There should be no rules. Hire Ted Koppel to moderate. Give Koppel complete authority to call out either one for not responding to a question, or giving a canned answer, and permitting one or the other to clarify if he deems it necessary. Koppel handled two or three opposing views brilliantly on Nightline, and I always thought he should moderate Presidential debates in that manner. It would make the debate more free flowing, and more substantive.