How the login details to a Genius Hub work
Each hub has a single username and password. This means they need to be set to something that the owner of the system is happy to give out to anyone who they want to be able to control the heating.
What if two people are logged in at the same time?
If two people are logged on at the same time, this is fine. If conflicting actions are taken (such as 2 overrides are selected) the Genius Hub will take the last command that it receives from any of the apps that are logged into it.