I see 2 position training as a reasonable way to initially build a team of decent players that can take you through early divisions. You can train 5 guys easily this way, versus the two to three for one pos. The money saved on buying equivalent players can be used to improve your arena.
But certainly, using two position training has a limit. Once you've got that base of players, you'll need to focus on guys and really push their skills. One position training is the way to go.