Actually, many sleeping bags are mateableyou simply buy one with a left zipper, and the other with a right zipper. But not all bags are mateable, because on some bags the zipper doesnt go all the way to the foot of the bag.
One advantage of zipping bags together is that you actually can get by with a lighter bag due to the gain from mutual body heat. My recommendation is to get a pair of Western Mountaineering Antelope MF bags. These down-filled bags are conservatively rated to five degrees, are extremely well-made, and weigh two pounds, seven ounces in the regular length. And theyre mateable. They cost $435 a pop, but theyll last pretty much forever.
In a zero-degree bag, Marmots Couloir ($400) also can be purchased with right and left zips and then attached to one another. Its a little heavier than the Antelope, at three pounds, three ounces each. But its a high-quality bag, with 800-fill down, a Pertex nylon shell with water-repellent treatment, a snug-fitting hood, and more.
Ive recommended down bags because cold-weather down bags typically weigh much less than their synthetic counterparts. But that isnt always the case. The North Faces Pyxis ($219) is a zero-degree synthetic bag that weighs a very competitive three pounds, one ounce. It might not last as long as the down bags, but it does cost considerably less.
Hope that helps!
The 2008 Winter Outside Buyers Guide is now online. From snow sports to trail-running to camping, get reviews of more than 300 new gear must-haves.