This is quite easy and there is no need for specific static routes for specific users.
- Befor you start, you need to have a primary/backup configuration for outside/outside_1 in place. I assume you already have this.
- Enable outside_1 in your webvpn-configuration
- Enable your trustpoint on outside_1
- point your DNS entry to the outside_1 IP
- Done
If you have a second name in your certificate or a second certificate, you can run AnyConnect on both interfaces in parallel to test it.
Or before changing the DNS in 3) you can point your FQDN on a test-PC to your second IP and test it there.