You can train both of them 100%. For example if you train OD, the optimal position is to play them at PG. To get 100% of the training they need to make 48 minutes at this position minimal per week. You can force 48 minutes on 1 player in 1 game and thereby need only 2 games to get both of them 48 minutes at the PG spot. This is the most efficient way and this way you can even train a 3rd trainee.
Alternatively you can have 2 games with 1 as a starter and the other as backup at PG and then the 3rd game / scrimmage play the backup as a starter and thereby have both get more than 48 minuten. This is less efficient since you can't train a 3rd trainee.
Read this post and the posts following that post to learn more about optimal training and even training in general:
(144856.30)Crunchy! If you eat fast enough