Set training to speed on the backup, no way he can chase down a QB even if he breaks blocks. Might as well start upping d-line general on the backup, as he has a good base to be a NT.
Set training to jumping on the starter, increases diving tackles, which essentially also makes getting sacks easier. Keep upping agility.
I like the way you are building both of them so far. It all depends in which direction you are trying to go with your builds. I myself have a NT RB Eater who has 90 strength and 40+ tackling. Now he currently plays in a 3-4 defense so doesn't get quite the stats he may, but he certainly dominates.
If you're building more of a pass rushing DT then I would say more vision is needed, alongside Shed Block in SA (which you seem to have done in your 2nd DT)
Otherwise you appear to building them very well so far.