Please note, doing this will double the size of your game install (since you are effectively installing the game twice).
To create your own Pendragon client:
Go into your root game directory.
For Labyrinth, this is your C drive, Program Files directory, Electronic Arts directory (c:\program files\electronic arts)
For other clients, this is your C drive, Mythic directory (c:\mythic)
From here, create a new folder named Pendragon.
Then, go into your normal game directory (Labyrinth, Darkness or Catacombs), select everything (control+a for short) and copy (control+c for short).
Now, go back to the root and into the newly created Pendragon folder. Paste the copied material (control+v for short).
To add a new shortcut to your desktop, right click on the camtest.exe file in your Pendragon folder and select Send To -> Desktop (create shortcut)
If you want to add the shortcut to your Start Menu, simply drag it from the Desktop up into the Start Menu wherever you like.
This now gives you the ability to play from live and test without having to patch each time you switch between the two.