Debian syntax for adding ipv6 static addresses to an existing (ipv4) iface (in addition to SLAAC ipv6 adresses (handed out by ICMPv6) which may not be guaranteed to be static). You may not use the eth0:0 alias syntax with ipv6 apparently!

iface eth0 inet static
 # feel free to do a "pre-up modprobe ipv6" if needed
 address xxx.xxx.xxx.xxx
 netmask xxx.xxx.xxx.xxx
 gateway xxx.xxx.xxx.xxx
 up /sbin/ip addr add dev $IFACE yyyy:yyyy:yyyy:yyyy:yyyy:yyyy:yyyy:yyyy/zz
 up /sbin/ip route add 2000::/3 via yyyy:yyyy:yyyy:yyyy:yyyy:yyyy:yyyy:yyyy dev $IFACE #this will route all presently delegated ipv6 addresses thru router yyyy:etc:etc

Where x = ipv4 , y=ipv6, z=mask bits for ipv6. feel free to collapse the ipv6 address of course.