Like orko27 said, don't mind what position your player is at; it's a mere suggestion the game gives you. If you are training two-position trainings like Rebounding or Jump shot, your player has to have 48 minutes combined on those two positions. Example: if you are training RB for C/PF, your player can play 24 min on C one game and 24 min on PF in next game and he will get full 48 training minutes.