The main theme in the lives of the Dioscuri is family brotherhood. These guys are totally inseparable. From the moment they hatch from their mother's egg, they do everything together: go on epic quests, steal themselves some wives, and even go cattle rustling. Unlike some brothers, there's no sibling rivalry here. Even in the afterlife, Castor and Pollux are together up in the sky where (according to most) they shine forever as the constellation, Gemini. From now until forever, the Dioscuri are always on each other's team.
Castor and Pollux are very similar in a lot of ways. As we point out in the family section, they're practically inseparable. The one major difference between them is that Castor in mortal and Pollux is immortal. Castor is the son of Leda's mortal husband, Tyndareus, while Pollux is the son of Zeus. Amazingly, this difference never seems to cause any tension between the brothers. You'd think Castor would get jealous or something, but nope. In fact, Pollux's immortality leads to a beautiful moment of selfless sacrifice after Castor is mortally wounded. Instead of letting Castor go down into the Underwold, Pollux begs Zeus to let them share his immortality. This selfless act leads to both brothers being given a place in the stars. In a way, the thing that made them different, in the end brings them together forever.