Not so fast -- the Treaze mfg, who seems an out front, authentic cottage industry guy following his dream, says he's about to release a lighter model,
called the FlyingBivy. Might be worth considering. I'm interested in at least lying in one to see how it feels for sleeping in comparison to my HH.
I don't much like the need to find a third tree, but this would be possible much of the time, I'm sure. For those hiking in rainy, highly forested areas, this kind of set up might be very appealing. It just depends on how the dang thing works. If it's extremely comfortable it might be worth the journey to getting it ultralight. It would be real nice to have some off the ground space for gear, and not have it roll under you every second.
Another idea is to remove the tent entirely, converting the bivy into an open sleeping platform. This would be the aerial equivalent of sleeping on a ground pad under a tarp when that's needed. Again, who knows on comfort? The jury's out. Give the guy credit for getting this thing out there though.
Best, Todd in Tarzana.