Is this something you would use?
Probably not, but I'm sure there are those out there that would.
Is a bag like this commercially viable?
Not unless on extremely limited runs.
How much would you pay for something like this?
I'm not sure, since i'm not sure i would buy it in the first place.
What features sound good?
Extremely light weight, sleeps two, feathered friends goodness.
What features sound bad?
The bag in general sounds perfect for a hard people of the world who get written up in Alpinist all the time. The Colin Haleys of the world. Unfortunately for this concept, I don't think there are a lot of people out there that want a route so bad they are ready to give up the sanctity of their own sleeping bag for the sake of saving some weight and share it with someone else. Those people are truly inspirational, but I think also think few and far between. It might be a great bag to sell to couples looking to shave pounds to hike the PCT or something, but I don't know that market.
What changes do you suggest?
I would design and manufacture a short run for the people (and a few others) that would buy it. Otherwise just offer them as a custom order.
hope this opinion helps.