Honestly, at that point I would just buy a cheap pair of earbuds, plug them into the Xbox controller (or use an adapter to fit Xbox's jack size, as I don't remember what it was) and use the earbuds for listening to Xbox. Put those in your ears, then connect your headsets to your PC and put them over your ears. Now you have complete sound dampening of outside noises while still being able to hear both audio outputs.
This is where I see the problem with using your headset for both. I'm sure you've loaded up a game on your PC before while watching a YouTube video or a stream and you have both sounds playing at the same time and it sounds like shit and/or is too loud and you can't understand what the hell is happening. Now pair this with 2 audio outputs. On the Xbox side, you will have the menu, game, movie, whatever playing into your headsets, then you have to account are you in a party or are people talking in voice chat? That's another sound being played. Over onto your PC, now you're having to account for any games, videos, streams or music that you are listening to. Now we're at 3 sounds at once. Then if you're in a Skype or Discord call, suddenly we're at 4 sounds that will be playing at once a good majority of the time, on the same headset. That is going to be way too loud, in terms of there is going to be noises trying to be louder than the other noises played back to you. If you get earbuds, you cut that in half per audio device which can help you differentiate between audio from your PC =/= game and voice from others | Xbox =/= game and voice from others.