In reply to frqnt:
im a huge fan of the double bag thing, having used it for 3 full winters (about 40-50 nights each, between 0c and -25c, up to 12 nights in a row), and a few trips to the big ranges mid year.
BEST option i think for bivvies is a -5c rated down bag inside a -5c rated synthetic bag with a bivvy bag only if conditions are nasty and a tarps not being used (ie if youve got the weather wrong and cant get down, or are simply hardcore).
ive never used a vbl, and find the function of a down/synth combo is all about stripping clothes off to get maximum body heat into the bag to drive moisture outwards into the synth layer.
over a night or two a down bag/synth jacket will be ok, but wont stay as dry as long. i wouldnt put a synth bag over a -11c rated bag because it would a) be heavy and b) too warm unless its really bullshit cold.
by example, the down/synth combo weights about 1200g - not bad for a -20c rating thats moisture managing so well. could get it lighter if your had the cash for top shelf bags.
it really depends on the length of trip. a single down + jacket and tarp/bivvy bag will be fine till moisture builds up. note that things like drying single boots inside your bag is a major factor, so with double boots you can go a bit further.
a light tent can be in the mix too, a bit warmer but therefore more moisture so the down/synth comes into its own..
worth considering now that single skin tents are down to 1300g, so for 2 people the equation changes.
really, with a down/synth combo and a tarp i feel confident to about -25c or serious wind and snow (where im fine with a tent, having done that to about 95kmph winds, 3600m, -25c).
hope that helps.
