Userdata on NAS

[mk01]

yes, it is possible but from many reasons it is not recommended for "whole" user data folder. there are some files (for example SQLite DBs) which don't support locking (simultaneous access from more devices).

what is most beneficial is:
addon_data (settings for add ons getting shared)
advancedsettings.xml
sources.xml
mediasources.xml
Libraries - but vi shared external SQL setup (MySQL) - which solves the SQLite share problem

if you have shared network folder, the easiest way is to create one, put it into /etc/fstab to get mounted during boot. and then just delete the original files (.xml) and create symlinks to files in that folder. This works fine with advanced settings.xml, shares, media shares, add-on_data directory and even Thumbnails.

I would not recommend sharing core addon directory, the .DB files, guisettings.xml files (as there are some unique parameters which will cause conflicts if used on both RPIs).

what is good, you can even share those files/folders with XBMC on other platforms (Win,Linux,Mac). NFS export for the shared folder would be highly recommended because of its native properties and locking and everything.