wonder if investing in a large carrier that fits both in would be
worthwhile.

I would put them in separate carriers if I were you; even though they get
along, sometimes if a cat is nervous and is in a cage with another cat, one
or both may get a bit aggressive and take it out on the other. I used to
work in a shelter and sometimes two cats that came in together would be put
together in one cage (large) and not get along anymore. You should find a
couple of crates large enough to put their litter box in with them. I don't
think there's anything wrong at all with travelling with them, they'll get
used to it once they've done it a few times. I know someone who used to take
his cats to his cottage about an hour and a half away each weekend in the
summer, they were fine once they realized they weren't going to the vet!

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