CASTOR and POLLUX
They are called Dioskouroi, which means "Zeus' sons" of which only Pollux was immortal. They watched after sailors and appeared to them as fires. Together they took part in many adventures. Castor was mortally wounded during a battle and Zeus suggested to Pollux that he shared his immortality with his twin. Thus each one took turns spending one day each in Olympus and one day in the Underworld.