I've seen double quilts but not double bags. I doubt it would be feasible at a zero degree rating where you need an excellent hood system and insulation all around you all the time. It should work for much warmer temperatures though.
Get two identical bags, one with a left-hand zip and the other with a right-hand zip. When it's time for bed, zip the two bags together and crawl in.
1) you each carry your share of the weight
2)you each have a sleeping bag with you in the event you get separated.
1)you have two separate foot wells since most zippers don't go all the way down.
(2) you have an uninsulated gap at the top of the bag between your bodies. you'll need to stuff a jacket or something there to stop the bellows effect and loss of warmth.
(3) you have no hood system to keep your heads covered. That means more bulk and weight for individual coverage solutions.
Having hiked and slept with my wife many times, believe me, the cuddling in a double starts out feeling nice, but you'll both really be more comfortable and warmer sleeping separately.