It's so hard to tell! I wish there was more solid data on training effects, kind of like hattrick.
I have found my players get too old too soon to reach the upper limits. So that's why I went for a level 6 trainer.
Yes, I'm training both PG/SG slots and spreading minutes to 5 players (which can be tricky).
Ron Ladner, my best player has reached 12-15 skill levels in all his guard skills - while training 4-5 guard candidates. That's not too bad, I just hoped for more.
Though I don't do too much 2-position training (except for the skills where I can't do single position training), what you described seems to be the consensus with 2 position training. You can build a very good team fast but it can be difficult to build elite players because you simply don't have the time to max out the players' potential.
Have you considered giving Ladner and Montenegro single position training for may this season and next? Higgins is 27 now and I'm planning to give him full training to at least end of next season. I hope those last few pops will elevate him from a good players to something special.