1) Get bags with full length zippers so you can use them open like a blanket in warmer conditions. That might be 50 degrees for the summer bag solo, or 30 degrees for the winter bag solo. As others mentioned, consider two bags for the coldest conditions, so the outer bag needs to be roomy to keep from compressing the summer bag. By using the summer bag as a liner, you do not need a full 0 degree winter bag. Maybe a roomy 15 degree bag will suffice. Loft is insualtion, so whatever keeps you warm at 0 degrees now is near the needed loft of the two bags combined. Don't just lay them one on top of the other, check the fit with them combined, then measure the loft. I shop for top quality used bags on eBay and check there once a week, year round. There are categories for down or synthetic and temp. ratings (+15 to +30 is one) to help you sort the listings. For winter sleeping when wearing more clothes inside the combined bags, the summer liner bag may need a roomier fit than most summer-only bags.