Plan A is better. If you go through with Plan B, you will make less profit since your players won't be trained as high as they could be. Secondly, training old players isn't a good idea unless they are, for example, 26 and have HOF/ATG potential. Older players have a significant slow down in training compared to younger players.
Two position training only benefits the player 75-80% of what they would've had if they were one position trained, if I remember correctly. The only time I would 2-position train is if I am training REB, 1v1, or JS. Just so you know, the player(s) has to play at least 48 minutes for the position of the skill you are training.
You can expect anywhere from 12 to maybe, 16 pops for a player who is 18 YO.