Before I bought my Sleepy Pods, I carried Mattie around in a soft carrier from PetSmart that was made by PetMate and was about $35 on sale. It was nice and light and I could just strap it over my shoulder like a duffel bag.
Its not a good idea to put two full grown cats in one carrier. Kittens are fine, but not adults. I'd get a soft-sided one with handles (not a backpack) or a medium size hard carrier. If the cat will be over 10 lbs, its better to use a hard carrier cause it feels more secure and sturdy to the cat.