I can't comment about traveling in the US, but when I travelled in Europe with my 2 cats, I was only allowed one cat with me in the cabin and one in the hold (except when the airline messed up and asked me to take them both in one carrier in the cabin). Also, if there is a dog on the flight, they do not allow cats and dogs to mix - so if there is a dog in the hold, they won't allow cats in the hold and if there is a dog in the cabin, then no cats in the cabin. No intermingling of species, I guess. Weird.