I never use a intermediate water bath when using my 2 bath development for VC (or graded) enlarging paper. I cannot see any reason for doing so. As far as mixing the two baths together in one tray, I think that would reduce the ability to contropl contrast and basically would defeat the purpose of using 2 baths. If the point is to creat an "intermediate" developer, for whatever reason, ok, but if it's tonal control you're after, then I say keep the two baths seperate.