Thing is Brad needs the support of the whole team, yet Chris has shown he is capable of going solo when the need arises. If they were both to ride as joint GC contenders it creates a fundamental split in the team tactics which others will exploit ruthlessly. There should always be a defined leader who the team will rally behind, then a clear order of succession if the leader bombs for any reason.
Imagine a scenario where the team is climbing at the front of the peloton. Brad starts to struggle so all the domesiques fall back to pace him letting chris go up the road with the lead riders, at that point he is isolated and the rest go hammer and tongs attacking one after the other to break him. Potentially both GC riders lose time.
I'd say you need a minimum of 2 super doms / doms for a GC rider (think porte and kennaugh) to pace and look after them, leaving only 3 other guys to do the flat donkey work and early stage pacing. Thats doesn't leave a lot of room for unforeseen incidents if you have to split the team in half for each GC guy.
Better to have one unified team all pulling for the same purpose. I can't see why, given the vast amount of events on the calendar, they can't both lead teams or even ride together as long as the rules are clearly defined by Sir Dave and each rider agrees to tow the line.