Here's the problem with having several distinct but related manuscripts on the go at once - when writing the discussion for one manuscript you keep thinking about how it relates to the cool findings of the second manuscript and suddenly your discussion takes a hard left and ends up in the second manuscript's jurisdiction.
Oh well, cut, copy, paste into discussion of second manuscript. Continue train of thought. Realize several paragraphs later that you have now tied this discussion into the cool new findings of the first manuscript. Which would be fine if anyone else were aware of those cool new findings. But they're not. Because you haven't published the first manuscript yet. Why not? Because you're still writing it.
Damnit! These things need to be published back to back in the same freaking journal so I can cross reference them myself.