Cats are super particular! You have two litter boxes, so one should always be completely clean! With some of the litters, it's best to leave the waste in there so it clumps completely. Raiden will use his a couple of times for pee, but he will always wait for it to e cleaned out for a poo. The only time it piles up is if he uses it overnight and I'm asleep.
Keep an eye on Zuki! IF he uses the bed again and again, then there could be something going on. If this was a one-time thing, consider yourself lucky. I hope you've taught your boyfriend how to clean the litter boxes as well.
Lastly, if you have two cats, you need three litter boxes.