I'm thinking the dual quarterbacks is not necessarily a bad thing. I remember when George Amundsen was a sophomore and Dean Carlson was a junior. They shared the quarterback position with one exception; occasionally Amundsen would come in as a wide out or something and generally a trick play would result.
The next year Amundsen moved to running back full time leaving the quarterback slot to Carlson. Amundsen became Iowa State's first 1,000 yard rusher. When Carlson graduated, Amundsen went back to quarterback. During Amundsen's All Star game (I think it may have been the Senior Bowl) he played quarterback, running back, wide receiver, and tight end to show his diversity. Both played briefly in the pros, Amundsen as a running back and Carlson as a quarterback.
If their isn't a whole lot of difference between Arnaud and Bates, I think the precedence at ISU is pretty good. I think a 60-40 split might work well changing the starter based on the competition. The following year if Tiller is as good as advertised, we could think about swapping out one of the three to their most likely pro position.