Yes it is true, sometimes it takes longer to receive pops due to 2 position training. Notice i said "Sometimes" , depending on the level of your trainer. I think 2 position trainer is faster with level 5 or higher, than it is with level 4 and under, though i may be mistaken.
Maybe someone with more knowledge can correct me if i'm wrong. Though at times, even when i had level 4 trainer and i did 2 pos training c/pf's, i think it was last season; It tooke about a week or two to get a pop. Overall though, its possible that the level of your trainer may make it faster to get a pop in 2 pos training. But yes generall speaking 2 pos is slow.
To be precise:
1. For any training regimen that has one position and two position options, a player will receive a larger skill increase in the one position version for any given training week. The two position option, obviously, allows for training of more players at a reduced rate.
2. For any training regimen other than free throws, stamina and game shape, a higher trainer level will always give more training for a given training week than a lower level trainer.
When you're comparing different players, even with the same level of trainer, same training regime and same amount of minutes played, their sublevels and the elastic effect can make a real difference on how frequently you see skill pops.