If an existing customer opens a 2nd account with the exact same title and signers, can we just reference the exisiting account number in the signature space rather than make the customer sign new cards?

You are asking a question that you need to ask your legal counsel as without a complete review of your deposit account contractual documents, it is impossible to opine.

Universal Signature Cards are common in CA. You might search that term.

Al

It's a common practice in credit unions, but it does not reflect a spur of the moment change of course. Their documents were designed to be used this way.

Thanks!!