If a player gets more than 48minutes it doesn't affect his training. 48minutes is "full" training and you can't get anymore than that.
If you have a player play 48 mins at PG and 48 mins at SG and you are training guards it would be exactly the same as if he only got 48minutes at PG. Anothr thing you might like to know is if you are training guards and a player gets 24mins at PG and 24mins at sg he will get the full 48mins training.
Hope that helps.