Networking to BR1's

I have a customer who is utilizing two different SCADA providers. There is a BR1 set up for each provider. The remote terminal unit they are connected to will only allow one gateway. So, I have to set up one BR1 as the gateway. This is no issue and something we do all the time. However, I need to link the second BR1 to the first so that it can also access the remote terminal unit. This is not something that I have done before, and I am not sure of the best approach. Can you please tell me the best way to do this and what I need to configure in each BR1?