Unless you're limiting use of a Back Country Blanket to 50+ F, I don't think it will work for two adults. Mine is narrow for me - I'm 6'4" - and works as a top quilt for two only above those temperatures or when mated with a second BCB. Length isn't the problem, as you can supplement the BCB with a sweater or jacket. You might consider NEMO's new double bag, the Tango Duo, seen at
http://www.nemosleepingbags.com/tango/ I was considering this when another forum member kindly sent me his Tim Marshall double bag. Of course Nunatak will make a BCB any size you like but that will cost quite a bit.