Oh dear! Just be grateful you are not there, otherwise you would have ended up clearing it up. That seems to be how it works in our house!
You probably need to permanently add at least one other box now anyway as they are growing. My two tend to use one for wee and the other for poop. And get the biggest litter boxes you can - we bought large initially but had to change to jumbo within a few months. Its amazing how much waste they can produce as they get older lol
Generally the advice is one litter box per cat and a spare. We only have two though and it works okay with regular cleaning up.
Your other half is determined to learn the hard way, isn't he? If I remember rightly he ignored you on the Dreamies front as well