I'll throw this in the mix.

My boss & his wife have United MP Visa deal.
(A) Joint Account w/MP Visa
Card #1 on this account. Wife's name on card. Her MP # listed on the card to accrue miles
Card #2 same account # as Card #1 Husband's name on card. Wife's MP # listed on card to accrue miles
(B) Second Account w/MP Visa
Husband has second card for work purposes (again United MP Visa). Different account number. Husband's name on card. Wife's MP # listed on card to accrue miles.

All miles dump into her account so she can horde them for family trips to France.

Typical scenario.
Husband buys UA tix on this card.
(1) Miles equivalent to purchase price go into wife's MP account
(2) Miles flown on UA go into his MP account
(3) Check-in @ Kiosk miles go to husband's MP account since this is not linked to the credit card MP # but the travelers MP #