Hi,
From the title of your post it looks like it is ASA Failover i.e IF active ASA fails, then standby should take over.
But from the description of the requirement, it looks like it link failover.
So i assume here it is link failover.
I assume you have 2 broadband connections on 2 different routers.
Refer below mentioned link which explains how to run HSRP for IPSec high availability.
HTH...rate if helpful..
http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6586/ps6635/white_paper_c11_472859.html