While doing his Fall camp opening press conference and media debriefs, Strong mentioned that while he'll likely have a clear starter named coming out of Fall camp, both QBs will have a chance to show their wares against the Fighting Irish in South Bend. He'd like to see what each can do under the lights and not just base his decision on an artificial practice environment.
I think that's really smart.
The truth is that most players can only do a fraction of what they show in practice in actual games, even though everyone imagines themselves to be a gamer. Some players can play exactly like they practice. Even a smaller number of athletes will consistently practice poorly but shine when the lights go on. I'm friends with some players from the James Brown era and he was pretty famous for practicing badly - at times, almost comically so - but playing pretty well when the lights came on.
Strong and his staff need to see if Jerrod Heard has a similar make-up. Similarly, given the meltdown at the QB position last year, they're not content to put all of their eggs in one basket, even if that basket appears improved.
As for actual implementation, game play will determine snaps. They're not going to bench a hot hand and I'm sure they have a few specific packages and plays that they know Heard is comfortable with that they'll be prepping him for and running. Think of an expanded version of the plan Bryan Harsin used for a true freshman David Ash.
If you want to understand how the Swoopes and Heard offenses may differ and understand the dynamics of the 2015 offense, I have a great resource for you.