Pandaria Alt with main

[Loaf]

is there any rule that does not allow me to use 2 character at the same time on the same acount?

[Anyone0]

If you mean being online on 2 characters at the same time then you can not do it if they are on the same account and same realm. The first character gets disconnected when you reach character selection screen on the same account on the same realm.

[Borrean]

This is only possible if you wish to play on several realms at the same time. If you wish to multibox you must create more accounts.