I don't use car seats but came across this all-in-one
dog carrier/dog bed/car seat when I was looking up dog carriers. It's got raving reviews, but I didn't end up buying one though. Wondering if this could comfortably fit two in it for your needs? It's 12 x 18 x 17 inches.
On amazon:
Amazon.com : Pet Flys Cheetah & Brown Reversible...
There are many colours too, from the Petflys site:
Brown Reversible Snuggle Bugs Pet Bed, Bag, and Car Seat in One