We have a customer who receives a check occasionally and it is made payable to their business and another. The payer's policy is to write the checks to both parties involved but we do not have a relationship with the other business. Has anyone else had this scenario? Could we receive something in writing from the third party with a signature and possibly a corporate resolution to verify signatures?

Thanks in advance.

The other party on the check may be able to take the check to their institution and get a "signature guarantee" or some such thing where they endorse the check and their institution certifies that the right person endorsed the check. Then the other party can bring the check to you for deposit.

The businesses, corporations, partnerships, LLC's, sole proprietorships, what? (It's possible you do not know what form of organization the non customer uses. That's an issue all by itself.)

Note that signature guarantees are not all that common any more and the other bank might choose not to do it. (I might do it once, but not as an ongoing practice.) Accepting it without the guarantee would be too foolish to analyze; you would be guaranteeing both signatures.

Plan B would simply be to refuse to accept the item for deposit... I don't know why anyone would write a check to two different entities, but their answer to "Why?" wouldn't make any difference to me.

sounds to me like a business trying to delay the clearing of a check as long as possible...

We had a customer who installed HVAC units in residential but also some large commercial buildings. He had to purchase units and other supplies from his vendors. Many of the large commercial businesses would write checks payable to his company and the vendor that supplied the unit. Our customer had a formal written agreement from the supplier that he could deposit those checks to his business account. Another example, is years ago we had a concrete driveway poured and to make sure the concrete company got paid, we made the check payable to both. (They came through the neighborhood and if we would do it when they were doing another neighbor, we could both get a better deal.) We called the concrete company and that's what they suggested. We did not want a lien put on our property due to the contractor not paying the supplier for the product. Just a fwiw.