And the two of them don't communicate with one another?
I hate to say this, but Regulation E doesn't address this possibility. My temptation (and yours, I image) would be to inform the consumer that you'll be happy to take the claim after s(he) has verified the transaction was not authorized by the other cardholder.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8