HA over L3-MPLS + WAN Load-Balancing

Hello,

I'm new to Fortigate and I need to design a new solution.

We have two separate buildings connected over MPLS.

There is site A site which has the WAN connectivity and the Fortigate HA and a site B with just a switch and the MPLS router.

We want a backup WAN connection at site B. I planned to create an HA with one FG on each site. Is that possible ? 

How would you achieve that ? 

Thanks for your help.