There is a question here, and a plea for your point of view - please read!
Having finally settled on the novel project I want to write - need to write, because it's been brewing and incubating for so long that if I don't tackle it now, it will be never; need to write, because it's the book that embodies the genre-blending (mystery in fantasy) I want to be all about - I had a horrible thing happen to me. I looked at an old manuscript (actually from a collab) and was blown away. We won't speak of the horrible craft in some of the writing, but apart from the fact that some facts needed to be foreshadowed or built up more firmly, earlier (naturally, because hey, I was pulling this stuff out of thin air) ... wow.
And I got so enthused, so delighted, in reading ... in remembering ... in finding side stories I had written and lighting upon alternate interpretations ... I can't remember being this gungho in a while, though granted I'm not sure if I'm just not remembering accurately. But I cannot find any downside, and that's a rarity. Some small concerns about cliche elements, but with this idea, I think I can fix those elements with ease.
I've thought about trying to write both books simultaneously (yes, I know, I know!) but the concerns are primarily: a) confusion of concept and b) I'd end up editing them both almost at once, and I'm already approaching that situation. I'm not sure that the first is an issue: they're both conspiracy / court intrigue concepts, but this is an area which I am so familiar and obsessed with that I think I might be able to better handle the similarity than someone for whom it might be two experiments. And in details, they are very different.
So for the writers out there: how many have you have seriously tried to write two novels at once? How did you figure out when to change between them? How did you keep the two separate? How did you handle the editing processes? Would you do it again?