2 servos are better, it eases the load that 1 servo would have and in most cases prevents aileron flutter which can occour on fast moving higher performance planes. For seting them up just make sure that they are mounted both facing the same way. Then connect the push rod on the outside edge of each servo arm. That way when 1 servo is pushing the control rod the other will be pulling the control arm. Just use a Y connector to connect the wiring. Your 4 channel will work fine.