Reading through several Wikipedia pages, and I'm still having a hard time understanding the reason that Robert was made Duke of Normandy and William II King of England.
A) Was this common, for two sons to each inherit some of the lands and titles, as opposed to keeping them all together? If so, when did it change to being first gets it all? If not, why was an unorthodox arrangement like this done? Or or were there different rules of succession for Normandy and England, each of which was properly followed?
B) Robert was the elder but only inherited a Dukedom, while younger William inherited a Kingdom. Was England seen as that much of a secondary concern, or was there something else going on here?
C) Why did only two children inherit like this? Was there simply nothing left for Henry to get, or was he purposefully not left something?
It was not uncommon at that time for the eldest son to receive the patrimony, the land inherited by the father, while a younger son inherited additions accrued by the father, even if the additions were larger than the patrimony.
EDIT: David C. Douglas, William the Conqueror, Univ. of California Press, pp. 360-1. Henry, son of William the Conqueror, got a substantial sum of money -- 5,000 pounds.
The treatment accorded to Robert and William might likewise be misunderstood. ... could be further explained by reference to contemporary circumstances. As has been seen, the succession of Robert to Normandy had been prepared by a long series of events, and it was not only his incompetence and disloyalty which prevented his succession to England. The Conqueror was here following the stablished practice of the Norman aristocracy which was that the Norman lands of a family (the lands of inheritance) should pass to the eldest son, whereas the English lands (the lands of conquest) should devolve on the second son. This usage had been very generally adopted [2], and it was a custom which in any event William might have found it hard to ignore.
But it gave Philip of France what he wanted, separation of Normandy and England.
[2] Examples could be multiplied. The instances of fitz Osbern, Montgomery Harcourt, and Montfort-sur-Risle come to mind.
Inheritance laws evolved. I do not know when primogeniture became the rule in England and Normandy, except that Henry II was willed all his parents' lands except for the traditional appenage willed to his brother, in the mid-1100s.