Can't find a link (can find tons of info on "B" Notices, but not on "C"), but here's a quote from the IRS's Publ 1335 "Underreporter Backup Withholding Questions and Answers" pamphlet that came with one of our C Notices a while back. It's dated 9/93, and has no corresponding link on their current website, but most of the info should still be good...
Quote:
Q: How should payers handle withholding if the notice from the IRS lists co-owners?
A: If the IRS notice to start withholding names several payees or co-owners, withhold on all accounts in joint names and any in which either payee is named first. If the IRS notice names only one payee, withhold on all accounts showing that payee's name first.
So, look for any account in that person's name, and set backup withholding on any you find, not just the account number they provided to you on the Notice.