Hmm, I think there is one minimum due to minimum proximity of stars.
Apparently this is distance is 9, which would make the minimum distance between two habitable planets (allways occur on the second ring around a star) 5.
This would require two stars to spawn within these 9 tiles of each other, both ahve habitable planets, and both habitable planets point in the right direction from the star. It's possible 10 is not a sufficiently high sample size to have it occur.
I'm (wildly) guessing here, but I ahve two candidates, which you could play around with:
MapSetupDefs.xml -> <PlanetFrequencyExp>
This apparently has something to do with the procedural generation of the map. Factorio has an explained wiki page on what their map options do:
This is one example of how it could work. It could also be entirely different for GC3
The other candidate would be:
I guess you will have to play around a little to find out.