My understanding of medieval succession is VERY limited, but my understanding is that in Feudal societies the concept of one person taking all titles was a late invention? Say I have two sons, and one is just TOTALLY incompetent, unfit to rule in my eyes. The other is a paragon of virtue and someone who I feel would keep the realm stable. How do I ensure my one son is the undisputed successor without resorting to drastic things like murdering the other one?
tl;dr how did medieval monarchs LEGALLY disinherit their children who they felt were unfit to rule.
Ah, you're William the Conqueror.
There really isn't much to say here. William's eldest son was Robert Curthose, but William didn't think he made a particularly good ruler. Although Robert was recognised as a skilled military leader (which he proved during the First Crusade, after he had been denied the throne of England), his administrative skills were deemed weaker than those of his younger brother William Rufus. Robert was also just lazy. Rufus was always the conqueror's favoured son, and Robert was often bullied by his younger brothers. The choice was pretty obvious: William the Conqueror wanted his son William to succeed him.
So he just told people that this was going to happen. When William the Conqueror realised he was nearing the end of his life, he sent William around England with a letter proclaiming that he was the new king, and if anyone had a problem with that then they could take it up with the dying (but still alive an in charge) William the Conqueror. Robert was made Duke of Normandy as a consolation, but William would become William II. In 1087 when William the Conqueror died, Robert attempted to stage a rebellion in the following months but he failed to show up to his own coup. He tried to take the throne again in 1101 when William Rufus died, but his other brother Henry was way more popular and Rufus' designated successor. Robert lost out on inheriting the throne not once, but twice. Power ultimately rested among the lords of the kingdom, and they would choose the person they liked or feared the most. A bit of diplomacy and effort was all it took to get William Rufus on the throne instead of Robert. The same happened again when Rufus was succeeded by Henry.
So although it could cause instability, it was as easy as writing a letter to lords and bureaucrats saying "this guy is my heir". As long as those lords and bureaucrats supported the claim, either through fear or genuine respect, then it would be fine.