I agree with the "Pair of GMs" idea.  That way, in the event of a sudden absence, there will be at least one other person that can direct where things are going.  Unless<em>both</em>up and vanished, but by that point I'd imagine the game would be in its death throes anyway.  As this thread is primarily about how to<em>avoid</em>such things, I'll assume the people in question are dedicated enough to stay on board, or at least pass on their knowledge to another player so that things can't grow aimless or stagnant.<br/><br/>