Unfortunately for your plan, while an audio signal can be
split easily enough using a Y-cable, you can't
combine two audio circuits like that: you need a proper mixer circuit. Sure, you could build one, but it would need some electronic knowledge: it's not just a case of getting the right sockets and wiring them up.
You might therefore find your only option is to buy a ready-made device. You are more likely to find something simple in one of the American catalogues - they like to listen to music as they go. Try
www.sportys.com.
That said, I don't think it's a good idea. Perhaps as a professional musician I find music more distracting than most, but I don't think I could manage another distraction in the UK's busy airspace (remember an American pilot can go for a long time without needing to hear a radio call, unlike here).
Tim