Which card is it? The AMEX Delta Platinum only gets the MQM credited to the primary cardholder. The AMEX Delta Reserve goes to the Primary Cardholder who can then either take them or give them to ANY other SkyMiles member. Important caveat with the Delta Reserve - you get a block of 15K MQM for the first and second $30K of spend. Each of these can only be credited to a single account, you cannot split the 15K between multiple accounts.

My wife and I each have our own accounts. I have a Delta Reserve and can give her my miles, she has a Delta Plat and the miles just get credited to her.

To add a litte more info. She has a secondary card on my Reserve account and if she uses it, it goes to my spend. I have a secondary card on her Platinum Account and if I use it it goes to her spend.

The key to what you do is based on your total expected spend for both of you and what you end up paying in card fees per MQM.