I've generally tried to keep my writing/editing work to a single title at a time.
The way I typically work is to rotate projects. For example, I'll write the first draft of a project (let's say Supply Chain, which I finished in February), then work on a proposal (Right-sized, for example, which is the next on up), and after that complete the third draft of yet another novel (Synergy, which I just completed today). During each of these cycles, I have only one project on the table at any point in time. Just one project on my mind. One set of characters. One plot line. One theme. One series of locations.
This method works extremely well because it allows both variety -- moving from one project to another -- as well as encouraging extreme focus by working a particular project-cycle all the way through from beginning to end.
The downside has been that it sometimes takes a long time before a book is completed. Years in most cases.
This year, I knew I needed to do something different if I was to achieve my goal of publishing three books (one, incidentally, is already out but is written under a pseudonym). So during this latest cycle (third draft of Synergy), I began working on the final proofreading edits for Pursuing Other Opportunities.
I worried that mixing the two projects together on a daily basis might cause confusion.
But it really didn't. In fact, it reduced writer's fatigue (my term for the point that comes along most days where I just want to put the project away for the night) by giving me something else productive to turn to when I needed a break.
Of course, final proofreading edits probably require less involvement in the project than writing a first draft. But now I'm intrigued. I'll have to try some additional combinations and see how the work!