Two ethernet and (one more) Docker network?

I have two servers, each with two ethernet ports. It is quite natural idea to connect first eth port on each machine to Internet (let’s call it WAN), and the other port use to interconnect servers (call this Backend network). Docker and Rancher uses its own network.

So the question is, what should I do? I’d prefer to use my backend network, but don’t know how to do that right way. Please advice!

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