Why are two wires used in railway overhead equipment?

If you think about this, the design is similar to that of a suspension bridge, and the curve formed by the support cables (the catenary wire in the case of rail electric feeds) is the same: a catenary arch.

We could send people and vehicles over the cables themselves. And in most cases, some people (usually bridge workers) do in fact walk along the cables.

But the suspended roadbed tends to trace a much flatter curve than the cables, which is more readily traversed by vehicles. A train's pantograph or trolley's contact arm similarly prefers a flatter contact path.

Summary: It allows the wire that touches the pantograph to be more flat

Allows the current carrying part to be copper, and the tensioning and/or friction part to be steel (or similar).