Route All Traffic Through Vpn If External Goes Down

I'm using Sophos firewall UTM9. I have 2 locations very close to eachother. Both have their own internet connection, and firewall. Both locations are linked with a point to point wireless bridge. This connection is what links the two buildings as we a lot of data between these location along with very large backup files.

I'm going to purchase a wireless modem to act as a fail over for one of the sites internet connection. I understand how to set this up and have it auto fail over if the external goes down at that specific location.

I want to know if say the other location without the fail over has their external connection go down, can I route their traffic over the VPN and out the external connection or fail over if both external connections down? The VPN between the two buildings will still be up as they are linked through the wireless bridge and not the internet.

Let me know if you have any thoughts or have any experience doing this.

Thanks!