Depends on task weight. If you're on a dedicated process then you're best off hammering it and keeping your momentum. This is where people want to put on the headphones and drone away. The longer you have the process in forefocus, the better chance to adopt optimizations, which just won't emerge without sustained sessions.
That said, there's also tasks that involve a lot of "click and wait" (incl literal clicking and waiting), that don't engage you much, that are usually unimportant anyway. Depending on the details and arrangement, you might see gains from multijuggling. If what you're doing is indulging (facetweet reddiblrgram socnets) on the side, probably not, but you probably don't care about any inflicted losses anyway.