My wife and I double under 1 tarp all the time, usually a 12' winter dream or 13' MWG Monster. Both are plenty big if you rig the doors correctly. We've done just about everything except the bunk bed style, and you definitely want separate hammocks/suspensions. Our preferred is the big tree (tough to find in our area) or better yet Y-tree on the head end, followed by the head end 32" spreader bar (from Dutch) through our beetle buckles. We never really have a movement issue unless one of us is getting in/out and then the see-saw effect kicks in. I'm ~300lb and lets just say my wife is a tiny fraction of that. The big issue is if she gets out, I sink and then she has to climb or jump back in her hammock. I'm not sure either of us notices much tossing and turning. No more so than sharing a regular bed anyway. Short a big tree or Y-tree, a medium tree with a well positioned sturdy branch can also perform the spreading function nicely. If I was just getting into this, I be getting one of Dutch's new XL Winter or Double tarps with pole mods. That extra width would be nice, especially on damp or cool days when you get condensation under the tarp.