Two characters (or groups) are intertwined by fate in a way that their luck is mirrored. For every good thing that happens to one, a bad thing happens to the other. Whenever one group rolls their dice, the game also applies the flipped result to the other at the same time.
There's a chassidic belief that there are only 600,000 souls and so everyone actually shares their soul with a bunch of other people like aspens growing out from the same root system. These people are connected at a spiritual level even if they never meet.