Site A has 2 WAN & Site B has 1 WAN, I need to create ipsec site to site tunnel with tunnel 1 as prioroty 1 and tun2 as priority 2, is it possible, the requirement is the secondary tunnel will act as a HA

Take a look at this article:

Hi ​@Jayakrishnan 

Yes it is possible.

Once the IPSEC VPN tunnel is created then you can set the route priority and AD value to make one tunnel as primary and other as secondary

https://community.fortinet.com/fortigate-3/technical-tip-routing-behavior-depending-on-distance-and-priority-for-static-routes-and-policy-based-routes-99660

or use SDWAN VPN for that. Only disadvantage I found on this yet is that this creates a zone for your vpn which is pretty nice for e.g.routing. Unfortunately a zone cannot be member of a zone in FortiOS which extends my policies...

Thank you all, For your valuable suggestions & advice. let me create  the tunnel and update you ASAP.