There are multiple mentions of "the dentist on the fourth floor". At least one exterior shot of the building shows a three-story building; there is no fourth floor.
I have assumed that they used stand-in performers to show the hands of the pianists, especially the Lizzy Moore character. In the "Jingle Bells" performance at the end, the last shot of her hands clearly shows only her right hand playing high notes with her left hand hovering above the keys, while the doubling of the melody in the bass notes is clearly heard.