If you want to maximise your chances of getting training for 6 different guys, you should dress no more than 8 players for each game. For both of your trainees, you have to assign them to all 3 depth spots (primary - backup - reserve). Also, make sure your bench players for 3 non-trainable positions are listed both as backup and reserve. This way you shouldn not have any vacant spots in your whole lineup. As others mentioned, use let them play.
You may also want to play a slower-oriented tactic to make sure your trainees don't tire too much (I believe some of them might have low stamina).
I've been using 2-position training for 6 weeks now and I've been fairly successful with that. The first 3 weeks I pulled it off and got full training for all 6 guys. The following 3 weeks I only managed to train 5 guys because of fouling out.
Good luck to you!