Only just. I've never read anything by Pratchett, so all I know about how their contribution worked out is from reading their interviews and essays at the end of the book. I think it's fairly evenly spread, although things like the tapes turning into Queen and Heaven having horrible music definitely sound like Gaiman to me. The naming and repetition of those names also feels like Gaiman, and the way the supernatural forces react and interact also seems very much like Gaiman, to me.
I read an article online (which I believe I posted in one of the biography threads) that had them explaining that they each took a set of characters and wrote their parts, then switched characters and wrote theirs, so that both of them had written a bit of every character. I also believe it said that at first Gaiman took the Horseman and Pratchett took the Them, but in the end they decided that they couldn't really say exactly how much time each one had spent with which character and so on. So although I see what you're saying about the wit being a bit different, I still feel Gaiman's presence very heavily, which, of course, is a great thing, for me. I will say this book is a bit more "Britishy" than other Gaiman novels, a bit more like Douglas Adams than he usually is, and that is where I would say I can definitely see Terry Pratchett's involvement.
That's as close to an answer as I can give, and I suppose a fan of Terry Pratchett might say something completely different.
Let me know when you (everyone reading!) get to the Horsemen, as I think they're incredibly well handled, their personalities perfectly fitting modern society, and brilliantly sardonically hilarious, and I can't wait to hear what everyone else has to say about them!