We got the 20 degree quilt. We've used it 8 nights so far and only 2 of them got cold - about 1 degree C above freezing. My wife has previously used the single 20 degree bag, and I'd say she finds this just as warm, if not warmer. Incidentally, we got the water resistant down, and also went for only 20% overfill instead of the standard 30%.
The pros are significant weight and space saving and reduced cost. Another possible pro is shared body heat.
The cons depend on your use. My wife is about 5" shorter than me, which means we don't fight for foot space, which MIGHT be an issue with 2 equally tall people, if they side sleep. Another con is that in warmer weather you have fewer venting options unless you both sleep equally warm/cold, since pulling it completely off your shoulders will affect the other sleeper. However, we've used it up around 15 degrees C without problems. The biggest potential con is the same as with double quilts at home; if you sleep apart, i.e. not cuddling, the quilt stretches between you leaving a gap at the top. When you move around, hot air can escape and cold air enter. We haven't found it a problem, and shared body warmth helps, but it might be a problem when pushing the temperature rating.
The quilt has a button hole in the centre of the neck so for one sleeper to completely close the top of the quilt around them. The second person can close their side and velcro it to the first person's (as in the 3rd photo on the webpage). We've stuck some of Joe's velcro tape strips (loop part) perpendicular to the long edge of our mats at neck height, and fastened a length of one wrap velcro to them. Then we slip a loop of webbing with a 1/2" triglide onto the one wrap and the triglide goes through the buttonhole to hold the centre of the quilt down to the mats and eliminate the gap. It hasn't been cold enough to need this yet, but it seems to work and should deal with the problem when pushing the temperature rating.
We're happy with the double quilt and won't be going back to singles.