SNMP Fortigate : bandwidth between two remote firewalls

Good morning, sir,

I'll tell you my problem:

I currently have two firewalls that constitute 2 remote networks that are not connected to each other (wired speaking), I would like to find a way to calculate the bandwidth at time T that there is between these two firewalls. However, both firewalls communicate through the internet, everything goes through the same output port.

How can I do this?

Thank you in advance.