The reason most split the elevon is for redundancy. With just two elevons, if one of the servos fails or linkage comes off you will have no control and crash.

By splitting them you could have both move together as elevons or possibly separate the elevator and aileron functions. Either way if one surface fails you would loose some control, but now have a good chance of getting the plane back in one piece. When you have $5K+ invested the cost of two more servos is cheap insurance.

Joe