H/D with two protagonists

I was reading a post today which linked to a discussion about H/D that got me thinking. I didn't actually read the discussion; only had time to skim it, so this isn't a real response to that thread. It just got me thinking. I believe the premise of the discussion was on the question of doms and subs in H/D, and which was which and why, but it got me thinking about ( The H/D dynamic in general and how it changes in HBP )