Reading SW rules it looks like they don't have a weight maximum per se. However, your cat must be able to stand and turn around comfortably in the carrier. There's no way they'll be able to stand once they're under the seat, but they might make you demonstrate that they're able to stand and turn around either at the ticket counter or boarding gate. I assume there are 2 of you travelling as you will need 2 seats. They also state that the carrier is counted as your carry on, but you should still be allowed a personal item. Sounds like most people use the soft sided Sherpa bags, if your kitties can fit comfortably in one of those you should be all set.
Good luck!