This should probably go in the suggestion forum, so one of the GMs will most likely mov this but this has been suggested before and has been and always will be shot down very quickly. If multiple trainers were allowed then too many people would just train 6 players and create a monster team from nothing. One trainer is enough. If you want your bigs to get pops from training your guards, place them at the guard spot or train your full team (which isn't nearly as effective but then your bigs are also getting training).
I think the game is strategic enough at the moment especially with the addition of the Game Day Prep. Adding more things can be good, but also keep in mind that we want to welcome newer members to the game and have them at least understand most of the aspects of BB. If we continue to add to many things, only the managers that have played for a long time will stick around, because BB may be deemed as "too confusing".
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs