I'm not familiar with futaba radios, but a 7C ought to have a built in mix for elevons. usually one servo is plugged into the elevator channel and the other into the aileron channel. Then you set the throws according to the model's instructions. "Differential" typically refers to the difference between the distance a control surface moves in opposite directions. In the case of ailerons it refers to the difference between the amount of up and down travel. Differential is most frequently needed on non-symetrical airfoils (Clark Y etc), but is often useful on biplanes. It compensates for the difference in aerodynamic drag between upward and downward deflection. I also use differential elevator throw if a model requires a lot of down elevator for inverted flight.
Hope this helps a little.
Walt