ISP failover is a feature offered by PIX/ASA version 7.2.1 or later.
This feature provides the option to configure a backup ISP route so that if the first route goes down, the second link automatically takes over to provide uninterrupted Internet connectivity.
This feature is called Tracking and it enables the PIX to continuously query and monitor a remote device/IP address via Internet Control Message Protocol (ICMP) (in this case a remote ISP's default gateway). If ICMP monitoring detects that the device is down, then a backup route takes over.
You can refer this link for configuring ASA failover with dual ISP
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_configuration_example09186a00806e880b.shtml