The North Face - Dolomite One Double

I have a North Face 2-person bag and I absolutely love having the extra space to stretch out. That said, even a quality 2-person bag (I think mine is 20° rated) is not going to be nearly as warm as a single bag or mummy sack. On particularly cold nights, I bring a fleece sleeping bag or blanket into the bag to wrap up in, and I also always have a large comforter to cover the whole bag. With these layers, plus long-johns, socks, a warm long-sleeve, and sometimes gloves and a beanie, I've been comfortable on some pretty cold nights. Sometimes I'll also make a nalgene bottle full of hot herbal tea right before bed and bring that into the bag with me. It'll radiate heat for a long time, and sips of a warm beverage will help as well.

I have the North Face Dolomite One. It's great for 3 season because you can play around with the temperature ratings. It has two layers on one side, for cooler temps, or you can remove a layer for warmer temps, and if it's even warmer, you can flip it to the other side which has an even thinner layer. Also, it's not a mummy sleeping bag. I can't stand feeling restricted so I have no interest in a mummy, but it's increasingly hard to find anything but mummy's these days, particularly if you are looking for something that also isn't super bulky (like the ones at Walmart that take up a ton of room and weigh a lot).