Imagine if you will a world like ours that is in a binary planetary system with another world just like ours about the same distance as the moon at least visually.
They both develop life and both develop a sentient, human-like species. They both evolve and progress at similar rates and for a long time they each stare up at the sky and see a blue and green sphere like we would the moon.
Eventually, they would develop telescopes and be able to look far close at these respective bodies and see cities and roads and canals and other evidence of a people like them.
But they would have no way to reach them for centuries, nor communicate in any meaningful way. Assuming both these people's on both these worlds are effectively just like humans, what would they think of one another? How would it change their course of histories respectively? Would space travel become a more pressing concern earlier? Would they live in awe or fear of one another? It's like the opposite of the discovery of the new world where you know there is an entire world you haven't discovered but are powerless to reach it in any way and can only watch and wonder... What would they wonder?