Non-resistant y-cables (wye-cables) are for splitting signals, not combining them. See this Ranenote for some of the technical details of "Why Not Wye"
Either a switch or resistive combiner (as was previously described by Gizmologist) are the correct & safe approaches. Commercially available combiners are commonly called 'DI boxes' and in my opinion to pricey for such an application. I have seen automatic switchers for video (composite) but don't recall seeing ones for audio, although they may exist.
Personally I use one of the little plastic project boxes from Fry's or Radio Shack along with surface mount RCA jacks, 10K resistors & a little bit of soldering. For an average DIY'er with basic soldering skills it'll take 15-30 minutes to put it together. It takes me about an hour to do one of these but I have visual acuity challenges that slows up the work.
If you do decide to do it yourself, here's a more simplified circuit diagram
than what the Ranenote had and it should meet your needs.