If you have 2 QBs, you have NO QBs.
The NFL is very competitive and Practice Reps matter, when you have QB Options to choose from, you end up splitting those Reps with the Ones (Starters) instead of one QB getting all the One Reps.
So you end up Under-Developing BOTH QBs, trying to figure out which one to roll with.
Which is why QB is the ONLY Position in the NFL where Competition isn't a good thing at... as long as you have a decent QB already.