I agree with Audioguru completely. One should NEVER use a "y" to combine two signals. In addition to what was mentioned previously, audio outputs are typically fairly low impedance sources that are expecting to see a relatively high impedance load. A good rule of thumb for easy calculation is that the load impedance should be at least 10x the source impedance and as you try to "y" two signals together this way, the load impedance seen by the source is much lower than it should be. The best way to accomplish this is with an active device that combines the two signals - similar to a audio mixer. There are some manufacturers that make two channel devices for this purpose or you could make one if you are so inclined. The next best thing is what audioguru suggested, using a resistor to passively combine these signals. Check out this application note for more info - it's pretty good, and gives some good examples of passive combiner circuits for both balanced and unbalanced (your RCA's) sources.