Norman:

Having parallel feeds of two outputs using a "Y" adapter is not a good idea, even when one of the pieces of equipment is off. Even in the 'off' state, there can still be some interaction from the inactive circuit devices in the output stage, causing at the very least some additional distortion, and useless loading of the active device. If you want to use a "Y" adapter, solder an approximately 10K ohm resistor in series with the hot (center) lead of each leg of the "Y". This will isolate the two pieces of equipment and prevent interaction. The only downside to doing this is that you must keep the interconnect length _after_ the "Y" to no more than about 3 - 6 feet at the most, to avoid high frequency attenuation from the capacitance of the cable and the 10K ohm impedance created by the "Y" adapter.