Is it possible using pay-to-script-hash, to come up with an appropriate script for mutually conditional transactions?

What I mean is, you have the following scenario: 

Alice and Bob want to make transaction A, but if and only if Carol and Dave make transaction B.
Carol and Dave want to make transaction B, but if and only if Alice and Bob make transaction A.

All four parties want both transactions to go through, or neither.  But none of them wants to structure it as a single four-way transaction, because Alice and Bob are not allowed to know before both tx are broadcast which txouts Carol and Dave are about to use, and vice versa. 

Nobody cares, though, if the other pair do instead a similar transaction (same amount transferred) involving *different* txouts. As long as it isn't exactly transaction A or transaction B, they don't want their own transaction to go through.

They can cooperate to the extent of giving each other identifying information for their transactions; they can give each other partial hashes of their tx (ie, their hash matches their tx if you zero the part of their script that contains your hash) before broadcasting the tx.