Tjin,

I am not clear on your needs. Do you actually plan on using this for shelter, or is it a backup for emergencies? If your plan is to use this and rely on body heat for warmth, I would strongly reconsider and take a sleeping bag.

Have you slept in a bivy before? They don't provide any insulation at all, so if it's cold outside, you'll still be cold. I really can't imagine a 2 person bivy working very well, I doubt either person is going to sleep very well. Maybe that's why there aren't that many companies making them.

If this is just for emergencies, I would recommend two separate bags so each person carries their own. There is no real weight savings to having a double bag, I don't think it'll be much warmer, and you never know when you might become separated or one persons gear gets lost.