[Senmut]

The Colony of Caprica orbited a binary sun, as evidenced by the view from Adama's ruined home in the pilot movie. Baltar looks out over the bay, as the suns come up. In such a system, more than one planet might orbit within the so-called "life zone". Also, if we postulate that orbiting furthur out from the twin suns, say at a distance similar or greater than the distance of Pluto from our sun, is another star of simialr spectral type, it could also have planet capable of supporting life forms. So, it might be possible to have Twelve Worlds, some probably moons of major planets, all withing a fairly small area of space.
Once the Colonials rediscovered spaceflight, it would only be a matter of time until they began visiting each other. Even for the most distant colonies, the trip would be possible before FTL was developed, if somewhat protracted.