Magnesium, Al, Zn strips would help protect the steel and copper. Magnesium stakes are sold for this purpose, and anodic aluminum is found in water heaters -- altho alum is proly not very effective after its own oxidized film occurs.
Copper outside form a protective oxide, like alum -- that green patina. So copper should be stable outside as well.
You don't need to protect the "steel AND copper." The copper is protected fine by the steel! Nor does the oxidized film have anything to do with it.
If steel, ductile iron, etc., is connected to copper, you have a battery. If they are connected again through soil or any other electrolyte, you have a circuit. Electrons will flow through the direct connection, ions will flow through the electrolyte connection. The ferrous metal will corrode while the copper will be protected.
If you look closely at outdoor connections you should see use of dielectric (insulating) components between valves, meters, etc. to break this connection. Copper grounding systems inadvertently connected to natural gas pipes can be a major problem.
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