I've done this many times, and both charges come through as separate dines. I've even had drinks at a place, then realized that buying a gift certificate would put me over a number-of-dines limit, and the drinks and gift certificate were counted separately, and I qualified for the bonus.

Unless, as others are saying, your dinner was also $15, on the same card, at the same place, in which case you may have to call Rewards Network to get it straightened out.