You can't configure this, if you define same source and destination for both tunnels with the router will always use the crypto that matches first on the vpn parsing, meaning the crypto map with the lower sequence number.
In your situation, you can use GRE/IPSEC tunnel on both tunnels and let dynamic routing handle the failover situation, having 2 different peers, both having same network behind, you can easily define a gre/ipsec tunnel to redistribute via OSPF or eigrp or any Routing protocol you need, the same network, and make the failover condition to happen by setting a preferred path.
An alternative to Ivan's approach is that you can specify multiple peers in the same crypto map entry so if the first peer goes down the second will be used. Basically the first peer to respond will be used -
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...