Then just pick one Sim and perhaps cheat up their skills?
Family play is a bit exhausting and not optimal when focusing on a skill or job with one Sim.
Somehow, my Miraj almost got fired from his medical career. His performance was abysmal and I don't know why, because I was also focusing on the other household members. To get his performance back up, I decided to have him quit and then re-join. It was easier that way.