What kind of bivy?
Long preamble, sorry:
My REI HooDoo 3 tent is rectangular, as are its relatively inexpensive "footprints" - designed to be an under-tent tarp. I think everything is roughly 7.5' by 7'. The footprints stuff into a small loaf of bread shape and have corner ties and grommets all over (I bought two).
I have the option to use any combination of the tent, the rainfly, one or two of the footprint tarps, a Thermarest mattress, an REI extreme lightweight bivy, a Wiggy's winter sleeping bag, and an REI 55+ travel sack (veryy light sleeping bag).
The lightest rig is to use one footprint under and folded over the travel bag, sometimes I use a pole or two to slant the part of the footprint over the travel bag up as a small lean-to shape. You could use the Silnylon Scout tarp the same way.
You could set the silnylon tarp up like a puptent or even a tube tent,, put the bivy down inside, and crawl inside just enough to remove the tarp-ponch.
I suppose you could reverse the roles of the two tarps, though I do not know why you would want to; especially if the rain were directionally consistent, you could pull the silnylon tarp over you, remove the tarp poncho under its cover, arect the tarp poncho, put the bivy down under the protection of the tarp poncho, crawl under and remove the silnylon tarp.
Another setup similar to one you mentioned requires setting up two "ridgeline" ropes, stacked vertically, and using two sets of tent pegs. one inside the other by about 6 inches.
Drape the smaller tarp over the bottom ridgeline at least 6 inches and secure to the ground on the long end using the inside pegs. and by rope to the inside set of tent pegs on the 6 inch overlap side. Drape the larger tarp over the top rope and overlap at least 6 inches on the same side as the longer side of the small tarp; secure the long end to the ground - to the outside pegs - and by rope from the 6 inch side to the outside pegs. What this does is create a vent-like space for air to get through but not rain, sun, starlight, etc..
Edited by dweste (07/24/08 08:41 PM)