Answer:
If you have a joint account with both a U.S. person and an non-resident alien, you should obtain a W-9 from the U.S. person and report payments accordingly. An unexpired W-8BEN is not required, as a W-9 never expires.
From the W-8BEN instructions:
"If you own the income or account jointly with one or more other persons, the income or account will be treated by the withholding agent as owned by a foreign person if Forms W-8BEN are provided by all of the owners. If the withholding agent receives a Form W-9 from any of the joint owners, the payment must be treated as made to a U.S. person."
First published on BankersOnline.com 2/04/08