On my last winter trip I was worried about the same thing, here's what I did.
Let me set the seen, it was in Yosimite Park the temps at night got down into the low single digits, me and my son were in a 2 man tent with the vestibules just laying on the tent, not staked out, I wanted to keep as much heat in as possible, but I also new this was going to create a lot of condensation inside the tent.
My solution, I made a syn. quilt the week before to take with me on this trip.
Inside the tent I was in my bivy I needed all the extra warmth I could get,
since I only had a 20* down quilt.
I put my down quilt over me first, then I put my syn. quilt over the down quilt and between the bivy, in the morning the syn. quilt was soaked, almost to the point that you could ring it out, but me and my down quilt were dry and toasty warm all night.
In the morning I just hung the syn. quilt out in the sun for about 30 minutes and it was dry and ready to pack away, I made the syn. quilt with black material for this reason, so the sun would help it dry faster.
Hope this helps.