Strangers has a perfect structure for a two part mini-series I always felt. The structure reminds me of It a lot: you have all these separate people who come together halfway the book (which is where the second part of the series would start) in one place and start sharing and dealing with what they once went through together, but forgot about.
I remember Intensity worked really well as a mini-series, but Strangers, one of his most mentioned and, I assume, popular novels has never been picked up for this format or a different kind of adaptation, which seems strange.
As far as writing, I don't think he slaves endlessly over it sentence by sentence (which is an almost impossible way to write), but he does many drafts of a certain page, completely finetuning it, which you can tell from the precise prose. And it's true he probably won't collaborate with King (or someone else anymore), as he says it's one of his flaws he can't collaborate.
Apart from that it's hard to tell if their styles would go well together or not. Despite all the comparisons I always felt they were quite different writers, certainly in style, but maybe even more so in themes, or at least the way they work out their plots.
There is obviously far more sci-fi in Koontz, but even when he does supernatural (like, say, The Mask) it ends up feeling very different from King. King is a horrorwriter mostly, I would say - you can always tell that's where his heart is, even when he does other things. Koontz is a suspense writer influenced by different genres, like sci-fi, crime, horror, espionage, adventure, detective.
Also there is a strong undercurrent of religion in Koontz' books (catholicism) which he works into his plots in imaginative ways. King doesn't have that.
I would say the most common factor with both is that their books are about ordinary, everyday Americans getting caught up in or threatened by something unusual, out of the ordinary. But for me after that most of the similarities end.