Let's say you had a solar system with more than one Earth-like planet innit, where intelligent life and technology evolved at about the same time in similar ways in both places.
Let's say it wasn't two planets, exactly, but that it was a planet and its moon, just to speed things up a bit and make 'em more dramatic.
So if you looked up at the sky of Either Earth, insteada seeing a glaring white ball of lifeless rock up there where our moon is, you'd see another blue-green planet up there, with oceans and forests and stuff, and maybe even the lights of alien cities.
Although mebbe you wouldn't know that you lived on a ball that looked like that, if you lived in the days before the abstract conceptual stuff like cartography became widely understood, when everybody thought the world was flat and stuff.
But eventually everybody would know that thing up there was another planet just like their own.
Probably a lot faster than it happened here, on Real Earth, where we didn't find anything super interesting (and potentially threatening to our survival) to look at like that even with our first telescopes.
I mean, having a clue like a Picture of the Earth From Space hanging over your head every night while you were pondering whether or not Your Earth was flat and the Center of the Universe and junk would be a distinct advantage, y'know?
And that's not even thinking about how it woulda helped folks here sort out junk like trigonometry a little earlier.
And then there's the Aliens over there on Other Earth to think about.
And we would be thinking about them all the time, trust me.
Whether they're having more fun than us whenever we're feeling bummed out, whether they're thinking about invading us and taking over, are they more advanced than us, are they less advanced than us, are they all gross with tentacles and stuff, or do they look just like us, are there Alien Spies from Other Earth Among Us?
I think it would have a pretty huge effect on our art and culture and religions, not to mention our personal and social psychologies and stuff.
And that's all happening long before we really knew anything about them.
And then there's the Old Standard thing about how the Potential Threat of Alien Attack would give everybody on our planet a common enemy to rally together around.
And advances in military and space superiority technologies would probably seem a little more important.
But at the same time, we might all be hiding in Underground Bomb Shelter Cities, relying on buried cables for communication because radio and television broadcast stuff would let the Aliens snoop on us, and that Two Planet Cold War kinda stuff would tend to hamper our ability to make and propagate advancements in technologies and stuff.
Scientists from Either Earth would be the first ones to communicate with each other, under military supervision, most likely, but maybe not always.
We definitely had the technology to completely destroy the moon before we had the technology to conquer it in person and claim all its resources, but the ability to communicate with someone over there would've come before either of those things.
Then again, all those capabilities popped up on Real Earth in very quick succession over the course of about fifty years or so, so there probably wouldn't be much time to make friends, y'know, before we started having to worry if they had nuclear weapons and stuff like we did.
And now you gotta remember that the exact same sorta pressures and stuff are happening on both planets at the same time, y'know?
Even if there is a lot of different ways you could handle it.
So in some cases of two earth-like planets in a solar system where intelligent life and technologies evolved at about the same time, you could probably end up with a tiny lifeless one that looked like our moon and another one that was sent back into a post-apocalyptic stone age after an exchange of weapons of mass destruction anyways heh.
And mebbe the guys that came afterward would never know that they had kicked their moon's ass a long time ago, and now they wouldn't be that interested in space exploration, 'cause there wasn't nothing out there to look at but a graveyard.
And they would tend to be sorta introverted, and fight among themselves, just like we do, 'cause there wasn't nobody else to fight with and there didn't seem to be nothing else to fight over, y'know?
And maybe they'll blow themselves up, eventually.
And maybe the Guys on the Moon knew that would happen all along, and when they were sure they were going to be instantly destroyed, they decided to make us die all slowly and horribly like this, y'know, making sure we stripped ourselves of our humanity and dignity first.
Or maybe we coulda been friends, and worked together, and shared all sorts of advancements and stuff, for a while, at least (heh), if only we woulda had a third earth-like planet in the solar system to rally together against.
Yah, like Mars.