I agree with Eric. I have several planes with multi engines including the 6 engine arado. There is no problem with the pickup reading the other engine's magnet. My planes all use Xicoy ECU and hall sensors which I presume you are using. Yes it is a good idea to put switches in both ecu leads from the Rx. You then can start each engine independently so if one does not start you can reset the ecu and try again with the other one running. Starting is by advancing the throttle fairly smartly so the engine already running does not rev up much. No need to switch anything other than the positive line. If you are in doubt try it on a test bench first.

John