From the description of the achievement it looks like every raid member must complete it during a single rotation and strike the same boss. It doesn't mention having to actually down the boss to complete it, so no clue as to whether that matters or not.
Logistically, this would mean that everyone would be stacking up and both bosses would be swinging at the entire raid. Wouldn't a hit from either boss on any raid member interrupt the completion of this achievement?
This would require perfect execution from the group (which is doable) and a huge amount of luck that the bosses do their attacks in the exact same pattern.
So, is this even doable? Thoughts?
Also, from what I can tell, no guild has this achievement yet. If someone does get it, how'd your group do it?!