2 Xbox in one house questions

Howdy, my son has a one S and my daughter just got the same for Xmas, trouble is I set it up via the app using the original login in my son has, but when my daughter plays a game on hers (even via disk) it kicks my son off and vice versa, how do I get around this? Is it because it’s the same login, do I need to add a new one? Any help appreciated

Thanks

:)