My son wanted to do a match on Halo: Reach with me. We have a Xbox One and a 360. I have downloaded Reach for the 360 to my xbox one. Is there a way to play each other over the wifi? We're not linked, and we are using the same Xbox live account. Perhaps that's the problem?
Halo: Reach should be compatible for both Xbox One and Xbox 360 cross play, as mentioned here.
Here's another similar question that might be helpful: Can I play against or with Xbox 360 players through my Xbox One?
I think the issue will be using the same Xbox Live account. If you are playing a matchmaking game, you communicate with other players via your Xbox Live account, and so if you are using the same account the devices will not know how to communicate with each other. I'd recommend altering your account setup, and considering using two accounts.