My wife and I use 2 MEC barrel bags zipped together, the nice thing about doing this is that they unzip flat, so one is the top and one is the bottom, they do various lengths and warmths. We have a 0 rated bag and a-10 rated one so depending on how cold it is we can just flip it over, thickest on top when it is cold, thinner on top when it is milder.
The cheapest option if you are prepared to make it is the Ray Jardine quilt.
The easiest option are the Big Agnes ones as you can buy them in the UK.
Nunatak do do a double quilt, it is mighty expensive, but apparently the craftsmanship is fantastic.
Another Rolls Royce option is a feathered friends barrel bag mated to one of their sleeved ground sheets, the advantage of this option is that they have hoods that zip on.
Feathered Friends have also brought out a double bag. again great quality but v expensive.