Then you almost have to set up a routing protocol like eBGP between FGT and Ciscos and iBGP between Ciscos (FGTs are already in HA). With BGP, you can relatively easily manipulate the priorities for each tunnel once you set a metric like community: 1=FIBER, 2=CORP, 3=RADIO for FGT advertisement to Cisco, and 1=RVPN1, 2=RVPN2 for Cisco advertisement toward FGT.
Then on receiving side, you can set local-preferences based on the community attached to each route.
I think you can find some examples for both FGT and Cisco separately on the internet.
<edit>By the way, SD-WAN on FGT covers just one direction or one side. You still need to set up something on Cisco side to choose one path over the other somehow.
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