Designing a Wireless Backbone

Hello everyone,

I am planning a wireless backbone for use in a "disaster" for my company. I have two separate buildings with Internet access (one w/ fiber, the other w/ cable). Building A has fiber internet and Building B has cable internet. I have multiple buildings with dual wan firewalls (WAN 1 is wired and WAN 2 is the wireless backbone and both go to building B for internet access). I want to configure Building B's firewall to send traffic coming in from the wireless backbone back through the wireless backbone to building A's firewall if WAN 1(cable internet) at building B is down . I was thinking I might have to setup a third WAN at Buildings C,D, and E to go to Building A if the primary WAN at building B is down or setup a third WAN at building B to go back through the wireless backbone and to Building A.I was wondering if I am on the right path and if their are other ways to go about this? I have also provided a link to a picture for reference purposes. In the picture, all the secondary WAN's go back to building B; I want Building B's secondary WAN to go to building A if Building B's primary WAN is down and I want Building A to go to Building B if the fiber goes down.