Servo controled by knob (wired) and remote control

Hello: I would like an electronic design to control a servo with a knob (wired) but also with one channel of a remote radio transmitter (like those used by model airplanes) It is important that the servo turn clockwise and counterclockwise depending on knob or radio transmitter movements, but in the same proportion, for example, if you turn the knob half turn to the left, the servo must turn half turn to the left and if the horizontal stick of the radio transmitter is moved halfway to the left servo must move half turn to the left. When radio transmitter is activated it must override knob, knob wont activate the servo when radio transmitter is working. I need a prototype and blue prints so it could be replicated, all parts should be easily acquired (standard) All system should work with 12v battery. Thank you