I have the answer, arpwatch can use only one nic at the same time, except if you launch one deamon instance for each NIC, see http://sgros.blogspot.fr/2012/01/arpwatch-on-multiple-interfaces.html
But the init.d script must be customised and become a new template if you want to keep it after an upgrade.
The same solution has been found by debian for the same purpose.
@dz00te don't be hidden...we wish only the best for you 