In the past, I've always worked on one book at a time. Then I'd set it aside and pick up another one. Then THAT got set aside, usually in favor of the first one although sometimes not. And so on.
For the last two weeks I've been working on both Dinosaur's planning and Blueberry's third draft. It's working amazingly well.
For Blueberry, I know the character and I know all her motivations and that sort of thing, so I'm focused entirely on the words on the page. Dinosaur is still at the "I know she does A and C but I'm not sure what B is that happens between them, and oh yeah, would she maybe do D instead?" stage, so it's the one I think about and obsessively run scenes in my head for so I can get to know the character. I think that's why it's working: I have one that's more structural/editing changes and one that's brainstorming and plotting.
But will it keep working? Up next will probably be the second draft of Cookie (again more structural, although second drafts are a LOT harder than third since a lot more changes) and the first draft of Dinosaur. They'll both require plotting and "why is she doing this thing?" brainstorming, and I'm not positive they'll mesh.
After THEM, if I continue with my plan, will be the third draft of Cookie and the brainstorming for Eaglet. But I usually like a gap between second and third drafts and I won't get it with this plan. And what if (if. heheh. WHEN) the two books don't reach the end of their stage on exactly the same date? Do I start the next one?
It's quite likely I'm over-thinking this. (Me? :) But I really want to ramp up my writing speed and release schedule in 2011 and I think working two books at once is the way to get there. I can't do much more than 2 hours a day of first draft or brainstorming work; my head just gets too tired. But I can edit for hours. So doing both should help make me more efficient.
I'll keep working on the order and the logistics of what ends when and what I do when one's done and the other isn't. If you have suggestions (even if they are, "Seriously, quit obsessing over this!" :) I'd love to hear them!