A few of our customers have two VPN sites or have two Internets to their VPN firewall in their office. They always ask to have secondary VPN tunnel as standby when the primary Internet connection fail. I can't find anything can make this work on NSX-T 3.1.3. Does anyone know how to make it?

When I am setting policy based VPN, NSX-T doesn't allow me to setup two tunnels that have same local/remote subnets, Then I think it is not possible for policy based. Then I try route based. I setup two tunnels, one for each Internet connection, they both are up. I setup static route on my T1 gateway to have two next-hop with different admin distance. but it doesn't work as I expected, When I shutdown the Internet on my primary remote site, it doesn't look like the traffic move to the standby tunnel, as I don't see traffic sending out on it. Where could I make wrong? I can't find any information about this kind of the request in the documents.