Wouldn't two pianists just sit down at the recording keyboard together?
That's the accepted procedure for 4-hand playing as opposed to two-piano
arrangements.  (I still don't understand how they recorded the
"Original Piano Trio".)

(Yes, I know that some teams play duets on two pianos; one player told
me it's less cramped on the bench, and when people come to concerts and
see two grand pianos they feel they've got their money's worth.)

Eric Stott