It is not a good idea to connect two line (or speaker) outputs together unless they are isolated from each other by buildout resistors (not an option to most consumers). Even when one component is "off", the output stage of the "off" component can either act as a short or act as a non linear rectifier, which will cause distortion in the component which is active. The resistors which are normally inline with the outputs of most equipment, and which establish their nominal output impedance, are too low of a value to have any isolation effect.

In short, don't do this type of hookup. If you're technician enough to do some fancy soldering, you can place a buildout resistor of 4K7 inline with the hot (center) leads of both legs of the "Y". This will eliminate the problem, but then the impedance is higher, so the output of the "Y" should connect directly into the input of the receiving gear.