These two from the faq relate to succession issues, hopefully they're helpful: How were twins treated under feudal inheritance law? Has the birth of twins ever complicated lines of succession?

I would recommend The Beaumont Twins: The Roots and Branches of Power in the Twelfth Century, by David Crouch. It's about Waleran and Robert de Beaumont, who were twins. Waleran was the elder, and there was never any doubt about his primacy.

However, it was common for Anglo-Norman noblemen to give their family lands and titles to the first-born, and land they had obtained themselves (through marriage, purchase, grants etc.) to the younger son, which was roughly what happened here.

A parallel can be found in the case of William the Conqueror, who gave Normandy to his oldest son Robert, and England to his younger son William (not twins, though).