Now, I could be wrong, and it may be that any coins stored this way must be fully spent when signed, instead of being able to be spent in parts (vault has to be emptied entirely, portion is sent to the recepient, remainder goes back into the vault maybe?)
Two-people-required-to-spend bitcoin addresses would be a great feature for blockchain.info.Coins would be sent to that address, then both address owners would have to agree to spend them (by signing transactions).
The incoming coins do have to be fully spent, but it is easy to create transactions that send the change back into the "vault" (multisig address).