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.
John S. Burnett
Fighting for Compliance since 1976
Bankers' Threads User #8