I recommend u to put all the back to the first seven. Because 1)i've seen that all the teams, which want to train more than 2 players on one position, they put them to first seven. 2) I do i myself and i see that even the other players in first seven train well.
Sure they will not get same training as players u have focused on, but they train better than in sub.
If the next match isnt such a important and u have enough good players to put on LP and PM, then try to put the not-focused backs into sub. Next game put them into first seven and check, when they trained more, after first game or after second game. I'm quite sure that they train better if they are in first seven.