The following is qualified by the fact that I am not a rebreather diver, but know people who are, and people who do "big" cave dives on RBs.
If you're diving a single RB, there is no point thinking about "thirds" or any other similar margin, as Stuart says. If you are going a long way, and deep (which to my simple mind is the main reason I might get into RBs), then it seems to me the only way to go is two rebreathers. Plenty of gas with variable O2 setpoint for deco, but probably some OC deco bailout for comfort factor (I always was a scaredy cat).
Marcus