it still sounds really REALLY stupid, if you don't mind my saying so. we do this almost every single day in the bank i work in and we can't figure out why your bank is being so difficult about this.
suppose any one of the payees had died? would they then insist that the instrument is now part of her or her estate?
what if under a different set of circumstances, you had inadvertantly made the check(s) out incorrectly--to the wrong person entirely? would they then insist that the orginal payee endorse the check even though they were never meant to have a hand in the transaction to begin with? what if the erroneous payee didn't even exist?
what if you were buying a car and you had simply changed your mind because you didn't like the terms the dealer was offering you? would they still insist that you surrender the check to the dealer--effectively entering into a contract against your will?
this bank is making this WAY more difficult than it needs to be and as soon as you get your money back i would advise you, in the strongest possible terms, to close your accounts and go elsewhere!!