You are probably not talking about an ASA-FO-system? There you don't need to configure two peers as the ip address will move to the secondary ASA when the primary fails.
If the two peers are individual boxes, then you can specify two peers in the "set peer" statement of your crypto map. If you use Pre-Shared-Keys, then you also have to configure a second tunnel-group for the backup-peer.
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...