However this also means that if you're setting a long term goal for your players to reach, that training time will take a few more seasons for your players, than 1 position training does. The older the player gets, the slower the training goes. So at some point it will even out and 1 position training will be better, however the question arises, when is this turning point?
I don't think it will ever be the case that you get the same number of pops (across all players trained) out of one positions as you will out of two position, which is what it sounds like you are saying when you say it will even out. However, you can only make players so good using two position training, so if you are trying to get someone up to superstar potential, you most likely need one position training.
It depends on the situation which choice is better, but I feel one position training is better in more cases.
For your team, and teams in situations you are most familiar with, absolutely, but for most teams, I feel like two position training is better. Remember, there are 16 times as many teams in D.IV as there are in D.II. These teams don't need to be training players for the national team (unless they think they can promote to keep up with his salary), they need a balanced approach that will allow them to improve all positions (by selling off excess trainees for cash to shore up their non-trainees).
My first season I did one position training (and with 2/3 of a season, my pops were not all that far off from 2 positions for a whole season), but after that I have been doing two position training. I plan on sticking with two positions for the next three seasons at least, and by the end of it, I should have two big men at 50k and 60k (and that's with spending two season training guard skills), which should be competitive in D.II, and even in the NBBA I could still start one of them (I would need crazy guards to start both though). Of course, this is just my estimation, maybe training drops off faster with age than I predict, I have only been playing for two seasons. (FYI, what I've been using for my estimates are 100% at 18, 95% at 19, 85% at 20, 80% at 21, 70% at 22, 65% at 23, 55% at 24, just numbers pulled out of my #$%, so feel free to offer better suggestions)
I think that unless you start off with two very good players you should be drawn to two position training right away. Yes, you need more trainable players, but the kind of players that will help you then are quite cheap. 25k-50k for a ~4k salary player with starter/star/allstar potential is what you're looking for. You can't afford a player with MVP potential unless he really sucks, and if you start with one you're probably better off selling him. Once you get those players up to their caps, then look at one position training, but until then mostly you need raw material to work with, and two position develops that raw material faster than one position.