Genesis

That's a bad scenario no matter how you try to handle it - but how realistic is it? A complete OOG at thirds either means (1) RADICAL inattention to one's gas consumption, (2) a simultaneous double failure or (3) two separate failures during the dive with the first one being intentionally iginored.

Only (2) falls into "act of god and not your fault"; the other two (which are the far more likely) are IMHO definitely your fault (the person OOG) and IMHO he/she is lucky to get half of your reserve in that scenario.

Let's say you close the isolator. He overbreathes his "half". Do you think he's going to go quietly, or try to kill you to get what's left, and do you think you can both fight him off AND exit on what's remaining?

If you're going to close the isolator you might want to think about it while he's leading and can't see you do it.....

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