I got one snuck onto my Kaos-D one of the trips it took back to the shop.
I have to say, the double frame is MUCH MUCH more comfortable on the hand.
The single frame and subsequent grip pads are much thicker and square edged than the double's frame and end up pressing very specifically into parts of your thumb as it goes around the marker. As it is less round, more of the pressure is placed onto a smaller area. I find it almost unbearable to use the single frame for a whole day. Though i did find that if i took the grip pads off, just the open single frame itself is about the same comfort level as the double with the grip pads.
As much as i like having a single, i miss the comfortable double.