Need more detail on uses. Does it need to be waterproof or
breathable or both? What price range, what weight?

Some have made their own using silnylon for the bottom and
a driducks poncho for the top to get a two person size,
waterproof breathable, cheap and lightweight bivysack.

It is for backup/emergency only. A sleepingbag is too big. The more sleeping gear i carry, the more likely i'll have to use it. I personally prefer single person models, but the guides want us to carry the double versions.

Requirement:
- Light weight and small (less than 500 grams)
- Waterproof, breathable is not required.
- Durable enough to survival 2 nights, but does not have last longer.
- proper closing opening
- 60 euro or less (~$70), shipping included
- vents would be nice.

I was thinking of turing two heatsheets in to a large bag and add a drawstring opening. I just haven't found a good tape, which i trust enough for the job. (to bulky, not sticky enough, to wide, loosing stickyness with heat and time)

Some make their own bivysacks from Tyvek. This is a housewrap
material used as a vapor barrier in construction. There is also
a Tyvek tape that holds very well. You would find it at a building supply or hardware store.

Some find it at construction sites where extra scraps are to be
had for free.

What did you finally decide on?

The two heatsheets idea sounded like a straightforward one to me. Before they had heat sheets type bivvies, I used to take two space blankets and tape them together on three sides to make my own emergency bivvy. I used clear mailing tape. It seemed to work fairly well. I wouldn't think being too wide would be a problem. I would think that being too narrow would be a problem.

HJ

There is always the Blizzard Survival Tube.

Weighs 540 grams
Cost around 50 Euros

I'd forgotten I had one,this might work-The Original Dreamie.They are made of a Satin type Polyester,& come in a few different colors,they weigh about 1 LB. & Measure-44"wideX 82" long,I beleive I paid something like $20.00 for it,Here is their website:www.buydreamie.com, Maybe this is something you can use,Good luck with the search! I have NO Affiliation with the above mentioned!

I like that design. Simple.

Some years ago, I sewed up a simple two person bivy bag, used primarily on SAR. I definitely prefer one two-person bag to separate one-person bags - although not toasty, you will be warmer with two heat sources inside a simple fabric envelope. I so used mine on several occasions, at least three times in very cold weather, and got through every night.

The construction was straightforward - Goretex on top, and coated nylon on the bottom. No zips, just an overlapping flap at the head end. Keep it simple and as light as possible.

Two person bags are not very common because there just isn't a mass market. On mountaineering routes, space may be an issue.