Writer Wednesday: Simultaneous Drafting

Today's topic comes from the lovely Mirka, who asked: "How many first drafts can you do simultaneously?"
I will be the first to admit that I can only draft one book at a time. That's just how my mind works. I need to focus on one story. Now having said that, I can revise one book while drafting another because I find revising and drafting don't tax my brain in the same ways. I can also plot one book (jotting down notes) while drafting another. Yes, those are similar, but I'm not actually drafting two books at once, so I can pull that off.
The issue for me is that I need to immerse myself in the world I'm creating in order to draft. This is also why I fast draft. I let the story fully consume me, and as a result, I can't fully focus on much of anything else. ;) 
For those of you who do draft multiple books at once, how do you do it? Do you have any tips to share for those who would like to multi-draft? (Did I just make that word up?)
*If you have a question you'd like me to answer from the other side of the editor's desk, feel free to leave it in the comments and I'll schedule it for a future post.