Configure existing IPsec VPN with failover to WAN2

Hi Experts,

i have two fortigate (200 & 100) which currently connected with IPsec tunnel.

We got a WAN2 internet on the 100, and my question is how to configure the IPsec tunnel to use the WAN2 if WAN1 has failed.

When i tried to set up another tunnel to the same 200 IP i couldnt with error: Unable to setup VPN: Empty values are not allowed

So i guess there is another way for such setup.

Can someone refer me to a guide on the matter?

Thanks in advance to any helpfull reply!