Forgive me if this topic has been explored before. William the Conqueror left his son William II England, and Robert Normandy. Henry Beauclerc was never expected to rule either, but ended up ruling both - one by conquest and one by right of survivorship.
Robert's son, William Clito/Adelinus was left the senior male heir to both England and Normandy after the White Ship debacle. However, William Clito died of a gangrenous foot wound at age 25 with no issue, despite being married twice. So, what if he had survived and left issue?