Redundant VPN configuration

Hi all in our company we have headquarter: Fortigate FGT 60D firmware 5.2.1 branch office: Fortigate FGT 60C firmware 5.2.1 Headquarter and branch office are connected with a Site-to-site VPN tunnel (used for PCs network and also for voip) both are using only wan1 but in headquarter we have a redundant connection managed by our ISP (so, actually, we have only one ISP connection) In our branch office we are installing another ISP connection (with a different ISP) so we will have wan1 and also wan2 I have seen that it is possible to create a redundant/load balancing configuration using the "wan link load balancing" but if I remember well, it is not possible to use it for a Site to Site VPN. Is it correct ? How could I do ? We need to have a redundant VPN configuration and also a traffic load balancing. I'd like that normally all internal traffic going to internet could go to wan2 and only vpn traffic goes to wan1 but in case of failure of one ISP the other one could manage all (vpn and internet traffic) Thank you