To additionally set up IPv6 after starting an IPv4 interface, you can create a skipt, e.g.
The script contains all addresses you want to assign and set the routes.
# IP1 ifconfig eth0 inet6 add fe80::2/10 # IP2 ifconfig eth0 inet6 add fe80::3/10 # Routing ip -6 route add fe80::2 dev eth0 ip -6 route add fe80::3 dev eth0 ip -6 route add default via fe80::1 dev eth0
You can check the created routes with
ip -6 route show