You may allow the customer to deposit any check that is “properly payable” under the Uniform Commercial Code (UCC). If a check is made out to two people alternatively (payable to John or Jane), then only one of the payees needs to endorse the check. But if a check is made out to two people conjunctively (payable to John and Jane), then both payees must endorse the check, and the bank may want to follow its identification, OFAC, and other procedures as to the non-customer’s signature (as is the current policy at your bank). 810 ILCS 5/3-110(d).