I just flew last night with both of my girls on Delta, we flew down to the beach for a long weekend.

With Delta, its a $50 fee one-way. If you have two dogs of the same breed and they can fit in one carrier small enough to fit under the seat they only count it as one dog, which is how I flew with mine! My girls start yipping if I leave them under the seat so I hold the carrier on my lap and play with them.
I used a Petote small Bali tote.