If a star system has two civilizations, what will happen between them?

At present, only the earth in the solar system is suitable for human habitation, but billions of years ago, the earth was still a rocky planet that had just cooled down.

Venus and Mars were then? Livable planet? They have atmosphere, liquid water and magnetic field, and they are wrapped in a blue ocean, just like the earth is wrapped in a blue ocean today, so many scientists think that Venus and Mars once had life in ancient times.

From the example of the solar system, it is not difficult to see that there are two or more stars in a star system? Livable planet? It is entirely possible, so what if there really is a star system with two livable planets in the depths of the universe?

The first case

If two planets are born independently and both show wisdom and civilization, sooner or later they will realize each other's existence, which will lead to a series of events? The third category? Contact, and the expansion of civilization and limited resources determine that the two civilizations will either jointly develop other planets, or they will not tolerate two tigers in one mountain and wage war with each other.

The second situation

Only one of the two livable planets has developed life and civilization, so after reaching a certain level of science and technology, this civilization will definitely immigrate to another planet for the first time and become a multi-planet species. What about emigrating to other planets? People? It is not clear whether it will become another civilization with abundant resources and the sky is high and the emperor is far away

The third situation

Both livable planets failed to develop intelligent species, so they stayed for a long time. Natural state? Hundreds of millions of years later, because of the increase in the luminosity of stars, the two planets were no longer livable and finally became lifeless planets.

For the current human civilization, emigrating to the moon and Mars is a guaranteed future, but we should also realize that the livable zone of the solar system will gradually move outward with the increase of solar luminosity, so the satellites of Jupiter and Saturn will also become candidate planets for future immigration and development.

The future of human civilization lies in the stars and the sea, not in the small earth.