Writing more than one book at a time

Until now, my writing process has been fairly straight forward:

 
Step 1. Get story idea.
Step 2. Write it and revise ad nauseum.


End of story (pun intended). I never worked on more than one thing at a time, because I  became obsessed with that one manuscript. This time around has been a little different. I've gotten excited about several different projects and couldn't pick just one. I know last week I confessed to a little summer slacking, but I've started working on three different projects. (Side note: as I tend to be an over-achiever by nature, my definition of slacking might differ from others.) They're pretty different in scope (boy-centric MG, YA ghost story, and YA horror).

I thought if I started on all of them, one would sort of "take over" and I'd finish that one first, but so far, I'm still drawn to all three. Granted, I haven't gotten that far yet but I wondered how that has worked for others. On the plus side, it gives me the opportunity to go back to something with fresh eyes pretty quickly if I've worked on a chapter for a different ms. One big negative is that if I keep working on all of them at once, it will take me that much longer to finish something.


Has anyone out there worked on more than one ms at a time? What did you see as the positives and negatives? Did one eventually take over? Am I crazy? NOTE: my lovely blog sisters are prohibited from answering that last one. :)