Sophos Firewall - Route internet from Head Office to Branch Office if BO experiences no internet connection

Hi Team,

We encountered a problem wherein customer needs the following:

1. Head Office will route their internet connection to Branch Office once they experience no internet on the site.

2. Vice versa, Branch office should route their internet connection to Head Office once they experience no internet on the site.

Is there any way we could set this up? We experienced this before in other vendors. Best scenario for this is like banks without internet connection, they get all the internet connection from the Head Office. 

Hoping anyone could help us out.