Exactly my thought. Once they get past kitten stage, there is NO carrier big enough for two cats. Mostly because one will sneak out while the other one is fighting about being put in. It's a tag team thing.

Depends on the sort of cat.
But I accept mine were the exception that proved the rule.
I bought two for my boys and only ever used one after the initial journey from the breeder (when I didn't know them sufficiently).

They cried less when they were together.
And they were very small by cat standards.
Well, not now - Diz is a big podge of course.

Also I kept the carriers out (partly because I had nowhere sensible to put them).
They had no fear of the carrier.

The only good thing was I was able to leave one for my lovely friend, who had limited funds and three cats. She did have three separate carriers, but one was quite old. The whizzy ones are easier to get cats into anyway.