I don't think Cutter retiring or co-chiefing would work. Cutter's got far too much history as chief. They'd look to him for leadership above Ember, whether or not he gave up the lock. And I don't think he could give it up either. I always got the impression that they split largely because the tribe was too big...the Shards war and hunting down Winnowill's creatures were convenient excuses. If it was meant to be temporary, he wouldn't have passed on the lock, and instead just appointed her as a temporary leader.