If one or the other struggles, I expect a change would be made. Frost doesn't strike me as the kind of guy who'll sit back and try to pound a square peg into a round hole (Riley.)
That's always tricky. Pulling a QB is not like pulling any other player in this or any other sport. There's some degree of perceived permanency in the move and that's made moreso when the backup is good enough to start in his own right. Deciding between what's just a bad day and what was the wrong choice in starter is not straightforward.
It's why I don't agree with the popular idea that one QB starts the opener and another starts later on. Frost will more likely, imo, give the starter every opportunity to succeed and that includes the errors a young starter will inevitably make.