Peter Gallagher actually does his own singing performances. Gallagher is a Tony Award winner & has done his own singing before.
Bruce Campbell is left handed but signs records with his right hand.
Borrows liberally from Neil Simon's The Sunshine Boys (1975), which also tells the story of a broken-up duo whose act is resuscitated by one of their adult relatives for a one-night-only reunion. Given the comedic nature of Simon's original play and the dramatic rendering of the subsequent film, it seems unlikely that the writers of this movie were aware of the earlier piece.