Had been busy with both work and house renovation.. sorry for the late respond..
The difference between one position training and two position training (besides the number of players that can be trained) is that players that receive one position training will improve slightly more than players that receive two position training. Some say that two position training gives each player about 70% of the training of one position training..
One Position Training is the most common training option for managers who are interested in quickly developing a few key players, for example if you wish to train a U21 or NT player ;P