Although Polydeuces was immortal, and of a son of Zeus, the brothers were as close as two could be, and never competed against each other. Together they formed what was called the Dioscuri, and are rarely mentioned as individuals. They were given the power by Poseiden to aid shipwrecked men.
They rescued Helen when she was kidnapped as a child (probably only twelve). Later, Castor died in a battle against two other brothers over a cattle dispute. Polydeuces refused immortality while his brother was dead, and Zeus allowed both to live on Mt. Olympus with the gods. They had joined the gods by the time the Trojan War broke out and were not involved.
http://homepage.mac.com/cparada/GML/DIOSCURI.html
