There is no easy solution to the "task switching" problem; it really is a strong function of how you work.
In my previous job, I had a rather distressingly large number of projects to work on—as many as eight or nine, depending on how you define a project—at any one time (oddly enough, my academic position requires me to work on just as many projects, but in a supervisory capacity). For me, what helped to have to "task switch" was the fact that I had to provide regular progress updates at team meetings, and meet deadlines for the project. I couldn't spend weeks on a single project, because then I'd have to scramble to meet the approaching deadlines for everything else I was working on. So deadlines and meetings where you have to talk about your progress are good ways to make sure you keep on top of things.
With respect to literature, I'd suggest setting aside some time each week for literature searches and literature reviews. I'd also use this as a chance to "switch gears" for a bit—use this time to keep up to date on the project that isn't taking up the majority of your time.
Finally, if you're going to put away a project for several weeks or more, then you should definitely keep good records before switching your focus. Make sure you leave yourself a note of what you just finished working on, where you left things off, and what you think the next steps for the project should be after you return to it. That way, you're not figuring out where you left off—you've told yourself how to get yourself back on track.