How to introduce PTP connection between 2 sites

Question

We're thinking of establishing a PTP connection between 2 buildings (about 200ft apart) using Ubiquity AirFiber. Primarily to use as backup internet for the remote office. Main Office - has 2 ISP's and LAN 
 Remote office - has 1 ISP and LAN 
 What's the best way to implement the new PTP connection on the Sophos firewalls in this scenario? Ideally we'd like to use the PTP connection for all inter-office traffic and have the remote office use its local ISP for internet. Then on remote office use the PTP connection for internet if its local ISP goes down. Any advice would be greatly appreciated. 
 
 Thanks.

BAlfson · Answer

In the Main Office, just a Static Route like 'Remote (Network) via PTP Interface' and a firewall rule like 'Main (Network) & Remote (Network) -> Any -> Main (Network) & Remote (Network) : Allow'. The PTP Interface would not have a default gateway 
 In the Remote office, the same firewall rule and, instead of a Static Route, Multipath rules (in order) like: 
 
 'Any -> Any -> Main (Network)' bound to PTP (with default gateway) with 'Skip rule on interface error' not selected in 'Advanced'' 
 Any -> Any -> Any' bound to External interface. 
 
 You might also consider a RED tunnel in addition to the PTP to have a backup for the PTP connection between the sites. Then, both would use Multipath rules to prefer the PTP (with default gateway in both sites) over the RED tunnel and all other traffic. 
 Is that what you had in mind? 
 Cheers - Bob