This answer is mostly on comfort, for more technical/practical considerations, please consult Rory's answer above.
I can express opinion about this double sleeping bag.
- Cheap, which affects it's other characteristics.
- Weights 3.6 kg, takes up most of the space in a large (60+)l backpack.
- Warmth is about the same as my single 3-season sleeping back.
- Putting insulation pads underneath is a little awkward, as the sleeping bag is quite large.
- Sleeping in the correct temperature (maybe down to 5 degrees Centigrade) is very comfortable. It feels like a large bed, in which one can move freely. Actually the bag can probably fit 3 persons.
- Because it is so spacy, there is some air movement inside (i.e. less warm than an individual mummy bag), which can be good or bad.
Overall, it is equivalent to two separate sleeping bags. On a serious journey I would not take it, as
- Finding even ground for two (actually more like three) sleeping bags is harder than finding two individual good spots to put the sleeping bags.
- None of the advantages of mummy-style sleeping bags.
- At the end of an exhausting day one rarely has the energy to hug the sweaty and filthy other person.
On the other hand, for leisurely weekend trips in warm weather this provides the ultimate comfort and intimacy with the loved one.