Fascinating. I couldn't resist doing a little research on this. Perhaps this will illuminate the possibility of JSS having done those two portraits for the ancestors of your mysterious inquirer.  

JSS was indeed in Asheville, North Carolina, in 1895.  
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Quote One  

From John Singer Sargent: A Biography by Charles Merrill Mount - First Edition (1955)  

From page 212 - (The year is 1895)  
 

Sargent passed through New York with Abbey, where Stanford White spoke of a portrait of his eight-year-old son, Lawrence, who previously had put his finger on a wet canvas in Tite Street. That never materialized. Instead, George Vanderbilt commandeered him with an invitation to his estate in the Carolinas, there to do portraits of Richard Morris Hunt, the architect, and Frederick Law Olmsted, the landscape architect, who were responsible for his magnificent establishment. Henry James, on a later visit to the estate, described it as “the extraordinary colossal French Chateau of George Vanderbilt in the said N.C. mountains - the house 2500 feet in the air, and a thing of the high Rotheschild manner, but of a size to contain two or three Mentmores and Waddesdons.”  Invited for May 15, Sargent was soon at work on two huge full-lengths, which he elected to do outdoors. The finished paintings were so striking that Vanderbilt had a three-quarter-length portrait of himself done as well. On the completion of his work in North Carolina, Sargent was bound for Spain 

From John Singer Sargent: His Portrait by Stanley Olson - 1986 - First Edition  

From page 193 - the page heading is 1895  
 

The pleasure earned in Boston was short-lived: three weeks after the first cheers went up, Sargent left for Vanderbilt’s new Renaissance chateau, Biltmore, in Asheville, North Carolina, to satisfy the commissions he had managed to avoid the previous year. He had been instructed to immortalize the architect of the park, Frederick Law Olmstead (1822— 1903), and the architect of the house, Richard Morris Hunt (1827—95) — two men worthy of immortality: Olmstead was responsible for much of the deliberate civic beauty ordained as some pastoral relief from all the building that was filling up Manhattan and Back Bay, and Hunt had shown himself to be the master of poaching arts he handed down to younger men like McKim. And when Sargent faced these two elderly giants he was further instructed not to believe his eyes. “My campaign here announces itself ominously,” he wrote [May 1895] to his friend Mrs j. Montgomery Sears in Boston, “— both wives prove to me that I must imagine thus that their husbands look at all like what they look like at present — totally different really, and the backgrounds, a stately garden for one and a venerable place for the other are at present red earth stuck with specimen vegetables and scaffoldings covered with niggers.”  Each portrait was, in truth, a double one and none of the four subjects was quite as he saw them. “Sargent arrives at his best by many substitutions,” James astutely informed a previous sitter,3  which was fine when it was a matter of subtraction, but at Biltmore he was asked to add as well, and both portraits suffered. It was an unfortunate reminder that portraiture was worse than enforced collaboration; it was vanity's butler. And it was also a fortunate reminder that he had got into a way of thinking about such employment as nothing more significant than a job of work, to take money.  The month he was in Biltmore his mind was still  in Boston; he was loyal to what he valued most. He wrestled with the welcome problem of where The Triumph of Religion might go next. He eagerly plotted his next course of study. Two days after leaving Biltmore, on 18 june, he sailed to Gibraltar. There he saw his mother, who was recovering from an attack of peritonitis, and a month later, with her safe retirement in Aix-les-Bains where she felt entirely at home, he headed back to London at a measured pace — “seeing as much as I can on the way”, he wrote to George Vanderbilt (21 July 1895) from Madrid.~ In other words, he, too, returned to a much-loved comfort — researching. 

“At the end of August 1895 he (JSS) began the quick march, to portraiture’s unrelenting cadence - reluctantly, very reluctantly” 

In the Charles Merrill Mount "Portraits in oils" list,  the following are found for 1895  

951 - Ada Rehan, signed. 93 x 50, Metropolitan Museum.  
952 - Mrs. Roller, signed. 88 x 43 ½ , Knoedler  
953 - Mrs. Russell Cooke, signed. 35 x 27, Mr. Hacker  
954 - Mrs. George Batten singing, inscribed: To Mrs. Batten John S. Sargent.  34 x 16 ½ , Glasgow Art Gallery  

955 - Richard Morris Hunt, signed and dated. Biltmore  
956 - Frederick Law Olmsted, signed. Biltmore  
957 - George W. Vanderbilt. Biltmore  

958 - Gardiner Greene Hammond, Jr., signed and dated. 28 x 22, Mrs. MacKinky Helm  
959 - Helen Sears, signed and dated. Museum of Fine Arts, Boston  
9510 - Eleonora Duse (unfinished). 23 X 19, Wildenstein, Scott & Fowles  
9511 - Mrs. George W. Vanderbilt (Mrs. Peter Gerry). Mrs. Richard Townsend  

Incidentally, there are no non-portraits in oil listed for 1895  
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Thus we can see that JSS was at Biltmore from May 15 to June 16 in 1895.  

During that time he painted three portraits; the Olmstead, the Hunt and the three-quarter-length portrait of George Vanderbilt.  He was there four weeks. Surely the first three days and the last three days or so - amounting to almost one week of time - would be taken up with settling in and packing to go - meeting his patron and subjects, admiring the chateau and general acclimatization. This leaves three weeks to create the three portraits for Vanderbilt.  Would he really have had to time to be driven into Asheville periodically and do an additional two portraits - one of which [your writer] describes as "a massive portrait of eight children" ? How would Vanderbilt have reacted to his guest leaving on a regular basis? As well, JSS was already losing his desire for portraits and was preoccupied with Boston; would he really want to do another pair of portraits?  

Frankly, I doubt if they are Sargent's work in spite of what "Birdie" says. And yet........... (I'd sure love to see images of these two works.)  

Do let me know if anything else comes of this.  

Hope you find something of value here.  

Be well  

Bert  

Subject: There were 6 total paintings owned by Biltmore estate 
From: Brian Hawley
B rian.Ha  wley@wachovia.com> 
Date :  Thu, 4 Oct 2001 

[Editor's Note -- Brian sent two wonderful letters, the first was on the 4th and the second was on the 8th. I have combined both meshing them together somewhat for readability only] 

Natasha, I found your fantastic site about Sargent and have been avidly  reading it recently.  I am currently doing research on George Vanderbilt,  and his amazing home Biltmore here in NC.  As I am sure you know he owned 6 Sargent portraits.  Can you give me more information about these oils?  So far I only have been able to learn who the subject was, and about the time they were painted.  Any help you can provide would be great!  I am  interested in the cost of a portrait such as those at Biltmore in 1890s dollars.  I have read the write-up you give to Sargent's time at Biltmore and enjoyed it immensely! 

The current portraits at Biltmore by Sargent consist of: 
 

1) Mrs. William Henry (Kissam) Vanderbilt 69" X 51 1/2" (his mother) painted around 1888 so I am told.  This I have noticed is not on your site.  I wonder if it was painted along with her daughters Margaret Sheapard. 

These next 3 were painted at Biltmore NC: 

2) George Washington Vanderbilt 42" X 26" painted around 1895.  This is the smallest Sargent in the home and I have always wondered why?  

3) Richard Morris Hunt, 91 1/2" X 60", the architect almost if not completely life sized painted 1895  

4) Frederick Law Olmstead also nearly life-sized 91" X 61 1/4, painted 1895, this one is slightly smaller than the Hunt portrait.

I am surprised you don't mention or have photos of these next two paintings.   

5) His Aunt Mrs. Benjamin P. Kissam (I don't have the dimensions) painted around 1888.  

6) His Cousin Mrs. Walter Rathbone Bacon 81  1/2" X 38 1/2" (These last were acquired I believe after 1896 but I am not certain of the date)

The Bacon painting is a favorite of mine with Mrs. W.H. V. coming second. Mrs. Bacon's portrait was also enjoyed by Paul Ford the novelist who was a good friend of George Vanderbilt.  On his visits to the home he would blow kisses to the painting!  George commissioned the Bacon painting which was completed in 1896. George acquired his mother's portrait apparently right  after her death.  I will send a photo have of the tapestry gallery taken in 1895 or 1896 that shows both his Sargent painting and his mother's on the wall. 

Also you mention the wellhead that Sargent painted Hunt standing next too might not have yet been setup or even shipped down from NYC since the final flurry of construction was going on.  However George Vanderbilt did own a 13th century unpolished Italian marble wellhead that is on the site today. 

It is also certain that the wellhead in the Sargent painting is nearly if not completly identical to the wellhead on the property today.   See the photo of me taken in a Hunt pose in the fall of 1998.  it is my belief that the wellhead in the painting is that very one still on the property. 
 
 

Brian Hawley as   Richard Morris Hunt  1998

Richard Morris Hunt    1895

 
 

You may know that George Vanderbilt inherited a HUGE three million dollar art collection from his father (William Henry Vanderbilt).  This collection was accumulated from the late 1870's until 1885.  After some financial trouble in the panic of 1907 George had to sell most of this art.  Sadly his father had relied on outside advice on the purchase of these works and by 1907 most were worth less than the original purchase price.  I surmise that George realized less than one million from the sales.  It is however fortunate that the family portraits and some cherished other works were not sold.  These exceptions to the sale obviously include the Sargents.  My point is similar to your essay concerning eBay and the possible Sargent  forgery.  I have always found it sad that William Vanderbilt did not pick up the fantastic works available in the 1880s.  Even with unlimited resources and good advice, art collecting it seems can be a difficult endeavor. 

Have you had the opportunity to go to Biltmore and see the Sargent paintings? 
 

Brian