I haven’t seen a true, two QB system work. Cal is running one right now, and they struggled to beat a bad BYU team.
Also, Shelley and Huntley isn’t a two QB system in my mind. They do the same things. Shelley is a better runner, but not significantly. Huntley is a significantly better passer. So, there would be no use in running it with those two.
Now, if Tuttle was your starter and Shelley the backup, I’d be ok with some “wildcat” plays with Shelley in at QB. Some redzone stuff, short yardage stuff. Nothing wrong with that, but I don’t see that as a two QB system. We are just playing Shelley instead of Weddle or Asiata or Booker at QB.