SD-WAN and IPsec

What type of ipsec tunnels are you creating ? Is it ADVPN HUB and spoke or is this a site to site vpn tunnel? Yes, for the failover purposes, it would be best to create two tunnels on each wan link i.e four tunnels but you don’t need to create 8 policies because you can enable multi interface policies. So if all these tunnels will allow the same phase 2 selectors, with multi interface policies you just need two policies

Hello Krissie, 

For redundancy purposes and SDWAN implementation yes you would need 2 overlays. For the next part it depends on what topology you would like to implement. If it's a sort of full mesh where every FGT is connected to every FGT then yes you would need policies for all the SDWAN zones you would create. Since the IPSEC tunnels will be part of SDWAN you will use the SDWAN interface in the policy. (not the tunnel interfaces itself)
If you are implementing a sort of HUB-SPOKE topology or ADVPN you can refer to the below documentation:
https://docs.fortinet.com/document/fortigate/6.4.2/administration-guide/783373/hub-and-spoke-sd-wan-deployment-example
https://docs.fortinet.com/document/fortigate/7.2.0/sd-wan-sd-branch-architecture-for-mssps/151899/basic-sd-wan-advpn-design

Hope this helps.
Enea