I've just tried changing Kai's cat litter from the Bio Catlet to the Catsan that Kim and barakta were using with Zev (and which she has no problem with). Since he hasn't cleared his bowels yet, and I've had one instance of inappropriate urination (followed by a significant purchase of carpet cleaner and air fresheners from Sainsburys), I'm guessing he didn't like that cat litter (or something else to do with the litter tray).
I think Kai's eating the wet food in preference to the dry, but both are premium brands, and he's still eating dry food as well. Actually for a small cat, who isn't doing much, he's eating quite a lot.
He's getting more vocal, but some of that is complaining when I close him back in the bathroom, and go away. Even when the doors open, he doesn't go that far from me, but I have to watch, and make sure he doesn't try and use somewhere else as the loo.
I don't think I'm even going to try and introduce them to one another until he's been to the vet, and been neutered. I don't want him leaving any more smelly patches, which the stress of another cat (even a kitten) could instigate. At least, once he's been to the vets, I won't have to watch him quite so closely, so he shouldn't need to complain too much about being locked up.