I did this before. When I did this I just called Delta and booked two seats next to each other. Not sure if it was neccesary but when we got to the gate, I explained to the GA again it was 2 seats not one and when boarding scanned both bps.

This was 2011ish so I really don't recall all the details. It was also in coach. 3 seats 1 in my name and 2 in my mom's. But I do recall we called to book.