I wouldn't be so certain about that. Professional Sleeping bags for low temperatures are designed to leave almost no room for air to come in to make the most of the body warmth.
Most of the single sleeping bags can be unified into one, but will significantly lose their isolation capability, because the narrow mummy shape is lost also, and more cold air can penetrate.
I really don't know how does the fact that two people sleeping together compensate for that lost of isolation, but the sleeping bag isn't utilized at it most.
The best thing to do is to buy two seperate bags which can be unified, and try it for a couple of nights.