How low can I go with this sleeping set up?

What shelter are you sleeping in?
Are there other people in the shelter?
What is the loft of the two bags combined?
What other clothes are you wearing?
Is the ground always covered in the 32 degree white stuff?
AND are you a warm, cold, really cold sleeper?

Well let's see, I would be sleeping in a double-walled tent or possibly the same tent pitched with the fly only. There might be another person in the shelter with me, but not definite. Combined loft would be about 7 inches, though I have never layed one bag on the other to verify that, I've only put one inside the other to check for fit, and yes, the ground is probably covered with snow. Also, I am a a pretty warm sleeper. I've taken both bags below freezing wearing only base layers.

I say try it on some cold nights then, probably with the full tent the first time. Start with expected temps of zero for a single night and see how you do. I am a warm sleeper as well, and I was just out a few weekends ago at -12 in a WM Antalope which is a 5 above rated bag, about 6-7 inches of loft. I was sick as a dog, and didn't eat much food for dinner so little sleep was had and I was moderatly cold but I am still alive. A few weeks later I spend 3 nights at or below zero in that bag and was fine. I would be confident that I could SURVIVE in that bag down to -20 with a full belly and my base layer on. If was going to be colder every layer I take would go on my body and then I would make it to the morning to pack up and walk home beaten and destroyed by the mountains.

My point is, give it a try. It won't kill you (within reason no -20 as a test night), you just may not sleep much. Make sure you eat a big dinner, take a few extra layers and more food into the sleeping bag with you so that if you wake up cold in the middle of the night you can put on more clothes and eat. As far as how to arrange them I would sleep zipped up in the heavier of the two and take the lighter one and lay it over you like a quilt push the down to the top on both bags and go for it.

Good luck! Hurry up and get on it before this February thaw melts all of our snow!

You have almost 20 ounces of 750 down in those two bags.

I have a NorthFace bag rated to 15 degrees with 19 ounces of 900 down.

Western Mountaineering has their Versalite rated to 10 degrees with 20 ounces of 850 down.

So this combination would probably be rated to 15 degrees.

Now if you can usually take a bag lower than its rating you could take this combo lower as well, obviously.

Maybe when you do bag in bag it will compress down more so it won't be that good

I think 15 degrees is what I am hoping to achieve with this combo. Has anyone ever tried the bag-in-bag method before? I'm interested to hear others' experiences.

I did once, the way I told you to try it. I move around too much at nigth for it to really work for me. The top bag that I used as a quilt ended up sitting right next to me by morning. I guess I didn't really need it.

The big thing is that the bags don't compress each other and eliminate the combined loft.

if the bags are the exact same, stuffing the one into the other has to compress the inner bag.

I cant say how much, and the advice of trying it will tell you for sure, but I would say at best with 3.5" of loft over you, you have a 0 bag (and I dont think you have the best case scenario) so maybe that 15 you are talking about.