I believe you have to go with your gut and follow what the work dictates. If one day you feel one manuscript calling louder for your attention, then go with that. But you might want to create a week long working process since getting into the writing can take some time to hear the voices.
But I have nine books in my writing queue and the order of the books is up to the inspiration. In-spirit -that's what inspiration is. It's letting the muses, work, words, or whatever it is have the power to give you what you need to create. So trust that.
Since my third book, I've always had at least two "in the oven." Right now, there are five (or maybe six).
I like being able to switch among them if I need fresh stimulation, or some days I'll feel like editing or designing more than writing. Also, often something that I write for one book, ends up in another where it may be more appropriate. Sometimes one book may suggest another, and it may become more important than, or even replace, the original. Sometimes books that were originally independent, get combined.
In short, be flexible, and don't be afraid to experiment.
If you have two incomplete books that have been sitting around for years, maybe there's a third book imprisoned in your head that needs to be typed before the others are published.
Michael N. Marcus
--Create Better Books, with the Silver Sands Publishing Series: CTRL + Click to follow link"">http://www.silversandsbooks.com/bo...
--"Stories I'd Tell My Children (but maybe not until they're adults)," CTRL + Click to follow link"">http://www.amazon.com/dp/0981661750