I have this cat carrier that opens front and top too:
Trixie Capri III Open Top Transport Box
I have tried getting both of them in the front door only type, but once one it is really tricky trying to keep him in while I get the other one in. And I normally need two hands to hold a wiggly piggie.
Opening the top its easy peasy to get them both in. Its sturdy too, and you can dismantle it for easy cleaning or if you want to use the base as a litter tray and the top as a tunnel. Big enough for the cat to go to the vet in too.