Wow! I guess I'm odd man out here. I only work on one at a time - that's what motivates me to finish so I can start on the next one. I do think about the next project a lot before I finish the current project
I guess I could look at this from a psychological point of view. Most of my life has been complex. I am now at the point where, as a widow, I live alone with my two cats. I purposely got a very boring job (used to have stressful job with large corporation), and tried to eliminate as much stress in my life as possible. Juggling more than one quilting project at a time would just feel like stress to me. Anyway, whatever works for a quilter is the way to go. Whatever floats your boat.