There is no doubt about it. Two campers can sleep much warmer when they are in a double sleeping bag together, than if they sleep apart in their own sleeping bags. All sleeping bags are built to isolate you from the cold air outside you, and trap your natural body heat to make sure you are warm during the night. The air is caught in the insulation in your sleeping bag, and is heated up by your natural body heat. This is what makes you warm and cozy in your sleeping bag. That is where you sleep warm from your own body heat.
With two folks sleeping together inside a double sleeping bag (or two single bags joined together) they automatically warm each other as well as the double sleeping bag; which means there is twice as much body heat to go around. That way you can sleep really warm and cozy, even on a cold night. Just snuggle together a little closer and grab all that body warmth!
There is another situation where you may need to put two (or even three) people together in a 2-person sleeping bag, or place them inside two sleeping bags joined together. That situation is an emergency that happens occasionally when hunters or campers who have planned a weekend or overnight hunt have been out in wet and cold and/or windy weather, and comes down with exposure or hypothermia. If this ever happens to somebody while you are hiking in the wilds or out mountain climbing, you must stop walking right away and pitch a tent to keep out the wind and the rain or snow.
Getting into a double sleeping bag at this point with more then one person can give some life saving warmth across to the weakened person. The person with hypothermia may appear confused, or wanting to fall asleep. That person's life is in danger because hypothermia is a killer! Keep them safe and warm inside the tent and sleeping bag, and send others to bring help immediately. If you have a personal distress beacon, use it now .
All kinds of outdoor sleeping bags are made in different qualities which vary from cheap, to top-quality camping equipment that will probably be in serviceable condition long after you have passed on. Be prepare to pay more to get a good quality sleeping bag, and you are unlikely to be let down. I suggest you take a good look at a few double sleeping bags in several camping stores and talk to the sales clerk or shop assistant and explain your requirements. Feel the loft of the insulation, look at the sewing quality and check that the zipper slides easily. Make a choice that you can live with.