Add the folloing to both boxes in the file /etc/networking/interfaces

BOX 1
auto eth0:1
iface eth0 inet static
address 10.0.0.1
netmask 255.255.255.0

BOX 2
auto eth0:1
iface eth0 inet static
address 10.0.0.2
netmask 255.255.255.0

then restart networking with:
sudo /etc/init.d/networking restat

then use the 10.0.0.IPs to use NFS