I've never faced this exact situation but here's how I'd be inclined to proceed...
1) Under "normal" circumstances, as confirmed at the 2009 Summit, a player has the "right" to bet-muck or check-muck at showdown. In which case the dealer is to respect the action, not expose the cards, kill the hand, and award the pot to the other player who needs not show his hand. Under these circumstances, IMO the hand is not dead until it's been killed by the dealer... at which time the pot is awarded to the remaining player. Clearly if a sequence can be established where one player tosses his cards first and they are first killed by dealer, then even if the other player follows suit, that player should receive the pot because the first hand has already been killed. IMO we might run into micro-analysis of this subject when we have, say, identically timed tosses but sequenced dealer kills or when Player 1's hand is tossed first but Player 2's hand somehow is killed first... but without delving into those variations which may or may not result in a different ruling but definitely add layers of complexity....
.... lets assume in this hypothetical situation the hands are both tossed and killed so simultaneously that we cannot establish a sequence of anything ...
2) So, first off I'm sure I would try and retrieve the cards which may be difficult if not impossible here. I would need 100% certainty as to the card identity to use this method.
Lets assume the cards cannot be identified to 100% certainty.
THEN, in the unlikely but hypothetical situation where all the above has occured, I would reason that the pot should be split. Reasoning is that we would award the pot to one or the other if we knew the sequence, since we don't we award Player 1 half for Player 2's muck and Player 2 half for Player 1's muck. Also, because these 2 players were the ones that took the risk and made the bets to get to the showdown, and because there's not a defective deck I don't like the idea of refunding to all players who have chips in the pot. I also don't like the idea of using an "order of showdown" rule here because bet/muck IMO supersedes order of showdown and makes it irrelevant... That's a way of saying that while I'm not excited about splitting the pot here, I like the alternatives even less.