I write both PB and adult nonfiction, but my current WIP is a YA.
I tend to let the most pressing project drive my work, while the other project "percolates." I like the clarity offered by giving myself time to think, and dream, about a project before I sit my butt in the chair and do the work. I jot down notes and thoughts (prompts) and put them in the file so the project is ready for me when I get there.
Sometimes I'll need a break from the work on an adult title, and use that as an excuse to work on a PB for a day or two. I find it refreshing to switch genres, and I come back to the original project with renewed enthusiasm.