On HQ , you will use apply crypto map on both the WAN interfaces and on Branch side , you will set the peers under crypto map as "set peer <Primary_ IP> <Secondary_IP>" along with tunnel-group created for both the peers.
Here are the documents that will prove helpful in configuring VPN failover:
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...