You dont need to make any sub-interfaces. Just use the same 'physical' interface for both failover and stateful links. It works as long as your state traffic does not overwhelm the bandwidth of the interface.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...