Hello,
This is possible only for LAN-to-LAN tunnels. If you are using remote VPN clients or any other VPN such as AnyConnect it is not possible.
If this is a site to site tunnel you just need to add a route point traffic for the remote network through the outside-backup ISP.
Example:
ISP1----------------------------
PC --- ASA === Site to Site tunnel === ASA/Router ---- Remote VPN client 10.10.10.10
ISP2-----------------------------
route outside 0.0.0.0 0.0.0.0 ISP1
route backup 10.10.10.10.0 255.255.255.0 ISP2
Regards,
Juan Lombana
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