Most guys run single 8411's on single elevons with no problems.

If you are looking for redundancy then the best choice would be twin servos on a single surface, that way if one servo fails chances are you can still move the surface and maintain control.

With split elevons you still have a single servo on each control surface. If a servo fails and locks up you still have a deflected surface and will have control problems, just like a single surface/servo situation.

I am guessing you have the Graupner kit which I believe uses the split elevons. I have the Jet Team Liebetrau model which uses a single elevon.