When configuring both IPv4 and IPv6 for an interface, the interfaces files should contains something like this:
auto eth0
iface eth0 inet static
address 192.168.1.1
iface eth0 inet6 static
address fdb2:c09e:ee01:1:1
I made a patch for Network to allow this, please pull the network-stanzas branch at http://git.ni.fr.eu.org/nicolas/propellor.git.