Ouch, one person, two cats. If they could fit in ONE carrier under the seat, that would be perfect. I'm sure a second international ticket would cost a fortune. However, if the cats are aware that YOU are there, it may be a much more pleasant trip. You might want to consider flying in off peak hours where the plane may not be full.
ColoVega has a great idea about subq fluids, but ... when it comes to giving them, the cats pee a lot more so that may be an issue. You want to make sure you have a lining in the carrier in case the cats do pee or poop. When cats are stressed, they can do this easily. Not their fault -- they are traumatized. And my DSH and Raiden both have pooped in their carriers on trips to or from the vet and it is not a great odor to enjoy for any length of time. Thus, you will have to bring baggies and paper towels or napkins and kitty wipes to make sure things stay clean. Thus, I would line the carrier with multiple pads, so you can easily take a soiled one up and have a clean one underneath.
This is a challenge, for sure. Know that cats do arrive okay in cargo but it would be so much better if you could have them with you. Airlines do restrict the number of animals allowed, unless they are service animals. Plan ahead is about all you can do.
If only driving across the pond were an option .....
I'm editing because I also want to mention that flying them in the cargo hold means arriving with all the luggage and baggage handlers are not careful about handling suitcases. I'm sure they don't care much about carriers with live pets. The carrier should have LIVE ANIMAL plastered all over it. Not that it would make a single difference to someone who wants to manhandle them. And the carrier doors should be double secured because cats can open them if they have enough time and energy. Thus, another reason for keeping them with you. They will arrive and get off the plane with you. You will not have to wait for them to come down a conveyor belt. If I had to consider moving abroad, I would probably not do it because of my cats. I'm sure, though, once you arrive safely and get settled, this will just be a blip on the memory card.