Yes, my regular super-cold weather rig is a down bag *inside* a larger synthetic bag (regtangular) You do need to make sure the outer sleeping bag doesn't compress the inner one.

It's definately not *lighter* than carrying one bag that is enough for your conditions though. I personaly use the above because

1) I'm too darn cheap[1] to fork out a grand for a nice WM puma.

2) For me, I do well with the above rig managing bag condensation in subzero temps.


As others have mentioned don't forget a warm pad - or two.. (I use an inflatable big agnes and one or two blue foamies.

[1] The kind of cheap that knows what it will cost me when the CFO finds out I spend a grand on the above bag..