As I already explained you can't multi-join a User in more than one Room concurrently.
If this is a use case in your game, join them serially: either by waiting for the necessary events, or by using a lock so that multiple threads will take turns.
I have provided clear examples on how to use both approaches.
To reiterate: each join transaction (for the same User) has to be completed before another one can be started.
Other than that joining Rooms is perfectly thread safe and concurrent.
I have seen something on the internet, which was not thread safe at all.
I have no idea what you're referring to...