Facebook launched new feature and now you can add many facebook ids on single device. You can use all those without log out any one and no need to frequent login.
In this method, you can directly add you ids in messenger and you need to click on setting option and choose switch account option.
Pop up window will ask you login credentials.
In second method, you need to visit android mobile setting and find accounts options where you need to choose add new account option and select facebook messenger option and put your credentials.
It is much easier, if you are not techie.