The issue I see is that if there is any kind of voltage or feedback loop in the switch circuitry of the machines, then having both the switches tied together will allow this feedback or voltage transfer to occur between them. I would be looking to make a circuit that utilizes optocouplers, or vactrols. The idea is to break the physical connection between the devices, but yet still have the switching control signal exist in any one of the two lines. I.E. the vactrols turn off one of the unused devices switch, thereby taking it out of the physical loop.