There is voltage involved , either 13V or 18V depending upon the polarity. If one receiver puts out 13V while the other puts out 18V, I can't predict what will happen, if the output are tied together. Voltage differences result in current which causes things to burn up. Some recievers, like the Mercury II handle their IF output pretty well , other recievers , like my CS8000 , appear to be very sensitive about what is attached to it. We can't generalize about any particular mfg's implementation of the If output circuitry.