I think I'm not understood here, so I'll break it down. I have 4 Sims in a band:
Hadrian: The Guitarist, mastered the Piano and Drums.
Charles: The Bassist, mastered the Bass.
Misery: The Drummer, mastered the Drums.
Rohan: The Pianist, mastered the Piano.
They've always been in the same household, and jammed EXACTLY on these instruments since the inception of the band. They all played the same instruments until Hadrian mastered the Piano. Then, he began playing the Drums instead of Misery, and he began playing the Guitar, even though he sucks in it. I will try kicking them both out, and then inviting them again, but I'm not sure.