Sorry for delay!
So I tested it on my end, the 2 tunnel goes up, but if I unplug one of my remote WAN port, the tunnel doesn'T switch to the other one (even if the tunnel is up…)
I configure the DPD (dead peer detection), 5 sec for 5 poll, to disconnect the tunnel, it doesn't work... I am not sure if it is possible..
I guess the only way would be to setup a DynDNS or NO-IP on the remote firewall so they can update the IP between the active ISP. But IMO, it is not a good solution for a large enterprise, as in my experience, for me, SOnicwall and DynDNS is scrap, no-ip works okay but I do prefer using a direct IP