There's probably a technical term for that, but that's what I call it.
Occasionally, I've found myself with two separate novels in the mental planning stages, and neither had enough to it to make a complete book. So I've played with the idea of combining them, even if they seemed utterly unrelated. In a couple of cases, the two separate stories clicked. I don't mean just that they could be forced to coexist, but that they seemed to be made for each other. I'd say that they illuminated each other, but that sounds too pretentious.
This is different from starting with an idea that contains various subplots. I'm talking about books that were conceived separately, based on unrelated ideas.
The first case that I'm really happy with is my last book, Business Secrets for the Stars, which I still consider the best thing I've written and which I'm now preparing for reissue by a small press, Norilana Books.
The second case is what will probably be my next book, and which I'm thinking a lot about right now but writing very little of so far, the working title of which is Chains. I assure you I won't let a sentence like that one survive in it.
If that works, I'll try smashing three unrelated books together! And then four! Wow! Maybe there's no limit!