A few days ago, I ran across an article about Keplar-62-e and Keplar-62-f, two planets slightly larger than Earth that are circling the same star (Kepler-62) and are both in the 'Goldilocks Zone'. The Goldilocks Zone is a band of space orbiting stars where the temperature allows a planet to have liquid water, and thus, it is assumed the planet could possibly support life as we know it.
Can you imagine what it would be like, to have another habitable planet in your solar system? In the case of 62e and 62f, their orbits are so close (closer than Earth and Mars) that one would be a little warm, the other rather nippy. It is speculated that the warmer one might feel like Hawaii or possibly DC in May, while the cooler one would be more like Alaska. I think we could probably find people willing to colonize both of those possibilities.
I have a story universe that deals with this possibility. I'm pretty sure other authors have postulated the possibility of 2 habitable planets in one planetary system, although I cannot name any off the top of my head. In my version, the inner planet is warm and fairly dry, kind of like eastern Colorado, and has an intelligent dominant specie. The cooler planet is more humid, with lush flora, maybe like Montana, and has no dominant intelligent specie.
The people of Planet 1 have no interest in space travel, and visitors from other planets are greeted coolly. There is no general interest in most of the merchandize offered by other planets. Occasionally an individual tribal chief might be intrigued by some trinket or another, and can be cajoled into exchanging a slave or two for that trinket.
But most of the civilized planets do not condone slavery. Accepting slaves is seen as a rescue, the former slaves are retrained and assisted to readjust to a new kind of life. And eventually, those former slaves gather together and colonize the cooler planet.
If I understand the Keplar numbering system correctly, there are 4 other planets in the Goldilocks twin system, discovered before these 'habitable' planets. Sibling planets of the twins, but assumed barren. Of course, there's always the possibility of life as we don't know it.