You might go to the TreeLounge website (just google it) and hit the FAQ, and look through the other material there. I can't recommend that you do something that is against what the manufacturer says,... but if it was me...
As far as harnesses go, I think just getting two should do the trick, with plenty of safety margin. I would probably use them as independent systems, though, just to make sure. That means 2 tree straps with one tether to each, as opposed to just using one treestrap with two harness tethers. You could probably rig up something so you only had to swing it around the tree once, though.
Like I said, ultimately it's your decision on how much risk you want to take, and no one here is going to be in the tree with you, or supporting you financially if an accident happens. Of course that goes for all of us as well.
Good luck with it.