yer i dress 9 players and set my trainee to Starter, Reserve, Backup.
The key is to also set your bench guys to be Reserve and Backup. 9 times out of 10, this works out.
I have had instances though, where, during garbage time, the trainee gets subbed for a starter, which makes me believe that stamina does have something to do with it. Trainne stamina is atrocious - this might be a problem? Not sure.
only dressing 5 players does guarantee 48 mins though. (except fouling out obviously).
but dressing 9 seems to work out for me most times.