I would have loved to have heard the famous Gottschalk/Thalberg 4 hand Verdi transcription.
Thal
You surely mean the "farewell"-concert at Niblo's Saloon!
Yes! It must have been unbelievable!

Sources say the following:
"[...]The farewell concert at Niblo's Saloon on December 26th [1856] built into a landmark event. Thalberg had already agreed to join Moreau, and since he was to depart shortly thereafter for New England, it became a joint concert d'adieu. To top it off, Gottschalk announced that he and Thalberg would premier a new "Grand Duo di bravura" on Verdi's "Il Trovatore", an opera that had been all the rage since its New York premiere in 1855. As the finale of the two virtuosos' New York season, this show-stopper offered thunder, lightning, and a hailstorm of arpeggios and tremolos culminating in the "Anvil Chorus".
"An extraordinary production", declared the "Times". In a seismographic report on the piece, Richard Hoffman announced that it produced "the most prodigious volume of tone I ever heard from the piano," especially the "remarkable double shake which Thalberg played in the middle of the piano, while Gottschalk was flying all over the keyboard.[...]"
(But, as sources say, even this wasn't their last performance as a team: they played, again, together in Albany, on 27th January.).-
Here
https://www.gottschalk.fr/index.php/en/gottschalk-s-eng/biographie-s-engit is stated, that the "Grand duo di bravura", which T. and G. played together, was
originally a version of the "Miserere" (
for 2 pianos arranged)....hm... . (thinking.., because: Did they modify it and played it on ONE piano (4hd), as the source seems to suggest, or did they play it on two pianos?)...

...however:
Of the "Miserere" itself there at least exists a piano solo-version, as sheet music (RO 171),
https://imslp.org/wiki/Miserere_du_Trovatore,_Op.52_%28Gottschalk,_Louis_Moreau%29and maybe a one piano 4 hd version (RO 172).
( Schott, 1878 ) .
The score, I think, then, to the (no. Corrected: a) 2 piano-version they actually had played, seems to be lost: That's really sad! At least I couldn't find it at the moment. But the work is listed as RO 267:
https://thompsonian.info/gottschalk-catalogue.htmlAnd Thalberg himself, as you, @thal, 1000% know, composed the following work which deals with the "Trovatore", too, and which looks mindblasting and awesome, so, I think, we shouldn't mourn, if RO 267 was really lost:
https://conquest.imslp.info/files/imglnks/usimg/9/9c/IMSLP07114-Thalberg_verdi_trovatore_op77.pdf Nevertheless, @thalbergmad: Please keep on celebrating!!
Best wishes for the future, and cordially, 8_octaves!!