How to start the twm manager on a system which already has a window manager ?

I am using linux mint codename rebecca with xface. I tried to follow a description how to start a window manager of once choice: 1. created an empty .xinitrc file in my homefolder 2. filled the file with "exec twm" 3. ran command "xinit" but all I got was a fatal error message and a message I did not understand: > (EE) Fatal server error: (EE) Server is already active for display 0 > If this server is no longer running, remove /tmp/.X0-lock and start > again. (EE) (EE) Please consult the The X.Org Foundation support > at http://wiki.x.org for help. (EE) Could you tell me what I did wrong or what I missed to do.