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I know how to mount NFS shares, this is explained in the Wiki and the forum.
What I don't know is how to first make certain folders (media/usb0) available via NFS to another RPi with XBian.

No other systems will be in this ecosystem, just one RPi running XBian for XBMC and the other one running XBian with deactivated XBMC, for downloading and BittorentSync only. The last one will have one or two hardrives connected via USB.

Most debian tutorials about setting up a NFS server will work for XBian as well.

yup I should have googled first and try that. I just found a very easy step by step wiki:
http://www.howtoforge.com/install_nfs_server_and_client_on_debian_wheezy

will try that after setting up the second RPi.