[Orgmode] how to change the set of agenda org files between custom agenda commands

[Rainer Stengele]

I am struggling to switch the set of used agenda org files between different 
custom agenda commands.

I tried something like

(setq org-agenda-custom-commands
      '(
        ("k" . "Kundenbezogene / Ketegoriebezogene TODOs ...")
        ("ka" tags-todo "CATEGORY=\"ADMIN\"")
        ("kP" tags-todo "CATEGORY=\"PRIVAT\""
         ((org-agenda-files (quote ("~/org/PRIVAT/PRIVAT.org")))))
...


or


(setq org-agenda-custom-commands
      '(
        ("k" . "Kundenbezogene / Ketegoriebezogene TODOs ...")
        ("ka" tags-todo "CATEGORY=\"ADMIN\"")
        ("kP" tags-todo "CATEGORY=\"PRIVAT\""
         ((org-agenda-directory "~/org/PRIVAT")))
...


but am failing miserably. Can someone point me to a solution?


Motivation: in the middle of my worktime I want to be able to look into a 
completely different (private) set of org files.
I know I could write a elisp function and redefine the 
org-agenda-custom-commands but I would like to do this in one place.

Rainer