Autonomus and Rc control of servo

Im working to an autonomous boat. I want to have a manual switch in case anything goes wrong with the boat in autonomous mode. But when I connect the signal pin of my servo to both my RC remote receiver and Arduino the RC remote stops working and when I disconnect the Arduino it works again. I've a simple program which reads in a value from the receiver and when it goes high it activates the arduino contol

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Ardupilot implements that type of function. It uses a 74LS157 multiplexer (four channels) and ATTiny (reading a fifth channel) to switch between the receiver and micro control. Look for the early versions and you should be able to find full schematics and code.