Claude, I have done the same in the past, it seemed to work well but I was much less experienced back then. I notice in the pictures that Christophe originally had the linkage wrong? I did go with 4 servos on my Reed Falcon biplane and am very happy with the results. It was quite easy to get the travels all equal and then after a few flights getting the differential correct as well.

Now onto more decisions. I am leaning towards a full flying stab using Pull Pull. Any thoughts here?