Wait until you guys turn 70...
I have one pair of glasses specifically for keyboards (organ and piano). As it happens, the piano keyboard (or organ keyboard) is at about the same distance as the music rack, so the glasses are optimized for the music rack. They are full vision (not bifocals).
If you are younger, and are having real trouble accomodating to distance, you should talk to an optometrist. If you are my age, you probably already have!