I've juggled multiple reads at times, but it's not my preferred mode. I will break up a particularly arduous project (like my current one: Plutarch) with some brain candy as needed, though. So I guess I'm a monogamous reader who dabbles in polyamory.

DD10 has scattered several reads together, many times starting but never finishing large novels or whole series, which I attribute to her naturally developing sticktuitiveness, which is age-appropriate but well behind her reading level.