They can exchange their views on height above the horizon, shape, color, and so on, and agree to call it, why not, moon. In short, two scientists just did what scientists love to do, agreeing on what reality looks like.
Next question: How far away is the moon? Two millennia pondering shows us that in order to be as precise as possible, we need instruments like telescopes and clocks, and a theory about the structure of the universe, Einstein’s general theory of relativity. And heavy duty math.
In short, the reality we want to find and agree on depends on things we had to invent in order to find that reality. In other words, what we see as reality is made by the mind. Scientists usually are aware of this dilemma and call this scientific reality a model – a model which approximates reality but is not reality itself. Scientists usually agree, models are the best they can do: Models are approximations, some better some worse. Einstein2: “As far as mathematical propositions relate to reality, they are not certain, and as far as they are certain, they do not relate to reality”.