The ASA doesn't support policy-based routing or load balancing out different interfaces. You can load balance to multiple next hops if they are out the same physical interface (i.e., two internet routers plugged into the same switch as the ASA).
Unfortunately the only thing you can really do is route tracking or ip sla as described by the other users.
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...