Georgia's setup is what I expect when I hear two-QB system. One of them will have to prove in camp that they will handle the brunt of the snaps. The second QB will have a package built to their strengths. This likely works best if Rivers takes the majority of the snaps and the "move" QB (Williams) has a package of plays built around his strengths. I am with you when it gets close to 50/50 is when one of the QBs has been ineffective and team is playing an unproven backup in an attempt to find a spark or a change of page.
This wasn't the question. I have no expectations that Rivers/Williams = Fromm/Fields. The question was whether a team can be successful playing two QBs. Georgia has proven they can, with the expectation that one of the QBs takes the majority of the snaps.