From the 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 that is a beneficial owner of a payment only if Forms W-8BEN or W-8BEN-E are provided by all of the owners. If the withholding agent or financial institution receives a Form W-9 from any of the joint owners, however, the payment must be treated as made to a U.S. person and the account treated as a U.S. account.
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