Only way is creating subinterfaces on a trunking enabled real interface which I assume you tried. ASA complains about the subnetmask issue because ASA can not have more than 1 interface in same network. You should assign different networks to each 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...