I know of very few banks that do not have in their agreements with such customers that the bank bears no liability to verify that two signatures are present. If you are still verifying such things, I hope you are charging a hefty fee for the service. You should have something like this in your agreement:
"We do not offer accounts that require two or more signatures. We may act on the oral or written instructions of any one signer on the account. Each signer may make withdrawals, write checks, transfer funds, stop payment, or otherwise give us instructions regarding the account. Any designation of a specific number of desired signatures on a signature card, resolution or other document is solely for your convenience and internal control purposes and is not binding on us."