Hi,
That is possible only by dialing with domain name to the ASA
I have configured and make it success to one customer like what you are expecting..Need to register one domain name like vpn.cisco.com, then you need to assign both your primary ISP IP address with the pointer value of 5 and secondary ISP IP address with the pointer value of 10 in public DNS server.
In normal scenario , the people used to dial with domain name to connect VPN, since your primary link is up the primary ISP IP adderss will be resolved by public DNS server, and the user can connect with Primary IP address.
If your Primary ISP link is down, that time primary ISP IP address will not be resolved by DNS server and the DNS server will resolve secondary ISP IP address.Then the users can connect the VPN with secondary ISP IP address.there will be no service impact to the users.
Regards
Karuppu