I recently had an issue where a client wished to route one IPSec tunnel over one ISP, and another tunnel over another ISP. One ISP was on the outside interface, and the other ISP was on an interface called backup-link.
I assumed, incorrectly, that it was going to be as simple as adding a static route for the IP of the destination to route through the second ISP's gateway. That did allow me to bring up the tunnel, but traffic would not pass.
The route I added was something along the lines of:
route backup-link my.external.address my.netmask my.2nd.isps.gateway 1
As it turns out, the ASA assumes that even IPSec tunneled traffic will be using the default gateway, so I had to add another route like so:
route backup-link my.internal.subnet.at.the.other.office my.netmask my.2nd.isps.gateway 1
And that seemed to work.