We fly only two or three times a year. When we do, we do so with frequent flyer miles accumulated from our credit cards, which get heavy use from my business. The Skymiles number as indicated on her Delta Amex was the same as mine, and the miles she's earned from her credit card are deposited into my account. She is also a member of the Delta Dining Club, and likewise, the miles she earns there are deposited into my account.

I don't really remember if we ever actually paid for a Delta ticket, so it may not be a matter of miles she is earning from a flight being deposited into my account. I also just checked our tickets from last year's vacation, and see that her award ticket did not reflect a Skymiles number because she wasn't earning miles from that flight anyway. (I may have misspoken about her using the my Skymiles number for flights, as I see that at the very least, that did not happen last year.)

So really what seems to be more of the case is that the miles she's earning on her Delta credit card, her MR points and her Dining Club miles, are being transferred into my account. It doesn't seem to really be a joint account, but rather, a case of points earned on her credit cards and her Dining Club miles being allowed to be transferred into my account.