i may be wrong but why couldnt one set up their hammock then the other set up theirs above or below like one normally would then lock the spreader bar between the said whoopies to just spread the suspensions apart the trick would be to have a way to LOCK the bar into the said whoopies so as one gets out that the other end of the spreader looses tention it wont fall out now as one gets in or out the other hammock would keep a specified distance " the length of the bar" away at all times. but when someone gets in or out the corresponding hammock will raise or lower slightly but should be minable "approx 6-8 inches " and the larger in diameter tree the less it will be i think that this whole thread is a great idea and im sure that there is multiple ways to do this. another thing is the spreaders would have to be fairly strong depending on the weight of the occupants. and with this style if the occupants are not close to equal in weight then the heavier will sit slightly lower. and as far as the tarp setup you could just do one ridgeline to set the tarp on and then set it up like you normally would but it would have to be wider than normal the length would be the same but would need to be at least 4ft wider "id shoot for 6ft if i was to do this but then again i like more of a covering". and if the hammocks used are the side zippered design then just make sure both of the hammocks are set zipper out. depending on the hammock "footbox or not" you might be hangin head to foot but thats a minor problem.
anyway thanks for reading my idea on this subject