Time Machine multiple Disks Backup over Network
So I managed to get a solution. 

You have to name your external disks which you want to swap with the exact same name (to be able to differentiate them just write different names on the cases). Then you insert one disk and share it as a time machine destination. Now go to the clients and select your shared time machine destination disk. When Time Machine has done its backup you can swap the external HD with the second one. 

Time machine on the clients will not notice that the destination drive has been swapped, neither will the server (it seems that the shared time machine destination points to the HD name and not as expected to the HD GUID).

CAUTION This workaround works best with mac os x 10.6, Mac OS X 10.5 will notice a swapped harddrive and show an error which references to a missing cookie or something in this kind. When confronted whith this error just proceed and make new backup on the swapped drive.