Yes, it does seem easier and a lot of people have done it (like the guy who built that Bear), but being an obsessive *******, it wouldn't feel the same to me, knowing it's a dummy. I'm really not doing it for gain of any kind, perhaps other than personal satisfaction.
ORIGINAL: Chevelle
With all the power available in today's R/C engines, the counter-rotating props aren't needed to fly the plane. It seems to me that it would be easier to extend the prop shaft, put on a dummy prop with a bearing in it that will slip on the shaft and then add the fixed drive prop. The dummy prop would rotate in the first props wash. If would spin in the opposit direction if the pitch was reversed. I've never done it but it does seem easier.