Yeah, it's clunky and inelegant.

Having the assistant gain-and-spend XP might work, although it leads to a few potentially odd cases. Still, might be the best bet. One thing is, when you take the advance, instead of the assistant just getting the new moves, they gain one.

Another option that might be a bit "faster" than tracking individual XP would be this:

Master Assistant (Requires Talented Assistant)
Your assistant has greatly increased their skills in their chosen profession. Choose 4 Advanced Moves from the Level 2-5 list for the assistant's class OR two Advanced Moves from the Level 2-5 list and one Advanced Move from the Level 6-10 list.