My experience after 6 seasons on the game, is indeed the 2 positions training is the best for a young team. Is the same route that i got too. Actually i started to train 1 position since the 2nd half of the last season, but again my team has great domestic success, and now i dont really need to fill any positions. I am on the process for upgrades. 1 player away to repeat my title, and i have plenty cash for that.
So in sum, if you are looking for team success your best bet is 2 position training. To chose 1 position should be 1)Have a player that can play on your NT, but you sacrifice the team development. 2)Be a team on a similar position like mine. Thats my opinion...