Just a related note that I found out the hard way once. Even if you have 2 cats that are littermates and have been together for life; put them in separate carriers when traveling. If they take a tumble while in the same carrier, each one will probably blame the other, and 2 cats fighting in one carrier is terrible. Plus, when you get them apart with only 1 carrier, you'd have no way to keep them apart other than leaving 1 out.
So, everyone has their own carrier. Every time I find them on sale, I'll buy a new one. We have 11 "pet taxi's" now.