Can't play on two Xbox's at the same time despite buying game twice

[Ares99]

I would like to play Call of Duty multiplayer with my nephew who lives with me. We have two xboxes. One is set as his home Xbox and the other is mine. I have GamePass Ultimate and he is a family member on the account. So that we could both play at the same time, I bought Call of Duty seperately on each of our accounts. However, it still requires one of us to sign out when we try to play. I thought perhaps he needed a seperate Xbox Live account which now appears to be called GamePass core. When I try to sign him up for that, it says he can't purchase it. Since i paid for the game twice, not sure why we can't play at the same time online. (We can play at the same time if he is just playing the campaign mode on his Xbox).