If you have a running QB who can pass
And
A passing QB who can run.... why not?
Who has that situation exactly?
Ohio State is the only team recently that had something like that with Braxton Miller(Runner who could sort of pass), Cardale Jones(Passer who could sort of run), and JT Barrett(Guy who can sort of run and sort of pass). That only worked out because guys got injured for the next man up to take over. It didn't work by switching them in and out all game for a season.
Alabama has 2 passing QB's that can run and 1 Pro-style passing QB red-shirting.
Of the 2 playing one is the defined starter with an 18-1 record battle tested and gets better every week and the other is the 0-0 back-up who gets better every week through quality playing time, but has yet to face a daunting situation.
Barring injury CNS is smart enough to keep managing the situation that way. Play the Starter....prepare the Back-up. Nothing more or less.
People from coaches to former QB's say repeatedly that if you have 2 starting QB's you really have No starting QB's. It's a unique position that does not lend itself to success with 'Co-Starter' status or equal playing time. It splits the team loyalties and is just a disruptive divisive situation.
When it's tied 17-17 in the 4th qtr in Baton Rouge(or AU, UT, MSU etc. take your pick) with 100K+ drunk corndogs (barners, cowbell clangers, hillbillies etc) shaking the foundation of the ground the HC NEEDS to know who is THE GUY.....and that QB needs to know that HE IS the guy....and the team has to know who is THE GUY to lead them. It's hard for all parties involved to know if you're flip flopping them.
Hypothetically.... you probably want the guy in there who's been there before and could stare down an oncoming train over a 'young-un' with a gunslinger mindset who could be baited into a bad throw with a disguised coverage.
Again just a hypothetical.