Two, definitely two! My two adorable nine-week-olds tumbled out of their carrier for their first vaccinations today covered in poo. Two carriers might well be an additional expense, but at least if one kitten is a bad traveller, the innocent party gets to stay clean!
Having a toddler you'd think I'd seen it all, but the state of those cats and that carrier took the biscuit. They are currently basking in a patch of sun recovering from their ordeal. I am basking on a patch of sofa recovering from mine.