I manage several concurrent projects with some of the same team members handing more than one work thread. From my experience, the more concurrent work threads the more likely individual contributors won't be able to complete the work on time. There's been several studies documented in two excellent books Slack and Peopleware (second edition).
I prefer limiting max work threads (good way to label the concept, Sandy!) to no more than three and if I can keep it to one (or at most two), everybody seems a lot happier with a higher probably to successful completion.
