Not with style, and not with voice, or viewpoint (when I want to do that, I often dip my toes into shorter pieces), but rather, with part of the process. I've been working steadily, and without break (well, okay, 4 days off, total, since February), on House Name (which I think will possibly have to be changed, as a title, but that's what I call it right now).
But I've also been working on an entirely different project at the same time. It's a novel, and the idea came up when someone asked me if I had a novel of a specific kind available to submit. Which is to say, the idea existed, and I thought it would work in that context. I wrote two chapters, and realized that it was not quite what they were looking for -- but I really wanted to write it, and since I occasionally daydream about writing a book for which there are no deadlines, I kept going.
I started it in April, I think, without much hope that I would manage to get it done, because I had no intention of setting aside House Name to buy the time to do it.
Once, way back in the dawn of time, I did try to write two novels concurrently. This would be before my oldest son was born (but not after, in which writing anything with the shock of New! Baby! was, for me, almost impossible). I couldn't do it. I tried, but I couldn't actually separate the story strands enough that the books didn't blend, and often I would be so much in one novel's headspace, I couldn't shift out of it.
So I assumed, at that time, that I was congenitally incapable of working on two projects simultaneously, and I didn't try again until this April.
Because the second project wasn't on a deadline, I felt free to fail. I also had enough writing time that I wouldn't miss a deadline on anything I have due this year, because the deadline work is work that I owe, and it has to come first.
The two projects are very tonally different books, so there's no overlap, and I don't find they blur at all. One is House Name, and the other is the first strictly contemporary long work I've ever done. The mood and feel of either book is distinct enough that there's really a mental divide in place between them.
What I've discovered so far--and I'm about two weeks off finishing the (much) shorter project--is that those long, hard, brain-dead struggles are not only, in fact, about my writerly state of mind. I can spend six hours squeezing out one thousand words on one book, and then when I switch books, I can write a thousand in an hour. It's more that some words, or rather, some tones, are much harder to penetrate, for me; it takes the six hours to sink, fully, into the right frame of mind for that particular scene. It shifts from book to book, depending on content, and I've come to understand, because I've tried this at all, that it's the content of any given scene, both emotionally and textually, that makes the writing a slog.
I think this has been obvious for a long time to other writers (matociquala comes to mind), but it's been a bit of a revelation for me.
The other thing I've discovered is that dual writing streams are still harder when I'm at the end stretch of a book, because that's the stretch in which the book devours most of my conscious thought, and often, given my dreams, a good portion of my subconscious thought as well. I think it slows down the end of the book, although it doesn't stop it dead.
I might write an end report when I finish, and finish editing, the secondary work, because I might find at that point that I'm wrong.