IMO 2-QB systems are underrated. The 2-QB system works out when:
(1) Both QBs are very good,
(2) The QBs have different skill sets that can create issues for the defense
It doesn't work out when you have two similar mediocre QBs, neither of whom is good enough to be a starter. The majority of times when 2-QB systems have been used, it fell into this category, and that's why 2-QB systems get a bad rep.
There's plenty of examples of 2-QB systems that have worked, though, with Leak-Tebow being obvious, but also Greene-Shockley at UGA, and Ainge-Schaeffer here.
I don't see much of a point of it though if two QBs have a similar skill-set, but it seems like Dormady and Guarantano have different enough skill sets that it could work.