[Blink]

Is it still possible to have a WM as a secondary boot up like, I can log out of DE and be into WM?

DE doesn't mean much, all it is is the applications for your WM.


For example, for the common desktop environment, LXDE:
Windowing System: X11
Display Manager (DM): LXDM
Window Manager (WM): Openbox
Desktop Environment (DE): LXDE, which is the following programs:

• LXSession (starts all of these programs)
  
• LXPanel (Application panel)
  
• LXLauncher (Application launcher)
  
• LXTerminal (Terminal emulator)
  
• PCManFM (File manager)
  
• LXNM (Network manager)
  
• LXRandR (RandR GUI)
  
• LXAppearance (Theme switcher)
  
• LXInput (Mouse/keyboard configuration tool)
  
• LXMusic (Music player)
  
• LXTask (Task manager)
  
• Leafpad (Text editor)
  
• Xarchiver (Archive manager)
  
• ObConf (Openbox configuration tool)
  
• GPicView (Image viewer)
  

All of these programs can be individually added to Openbox (like in my setup), and are easily replaced by other programs (Tint2 replaces LXPanel in my case).
You generally don't 'log out' of a DE.  You can do so with a WM if you are using a display manager (otherwise you'd need to log out of the shell too), but logging out of a DE would just be exiting all those applications at once, which doesn't do much except leave you without a panel.

Ah right. Because, I'm running Arch xfce DE and wondering how I can login into i3 or some sort of WM.

XFCE is XFWM + XFCE utils.
Most display managers (LightDM, MDM, SLiM, etc) allow for easy switching at the click of a button. Just log out, and switch in the login menu of the DM.