Sure, it doesn't reflect the exact (and very muddy) situation IRL, but I think it's the best approximation without introducing complexity that doesn't enhance gameplay. I think these situations are already reflected sufficiently in the De Jure title laws, that even though the Dukes of Normandy were also the Kings of England, the Dukedom of Normandy are still de jure vassals of the French Kingdom.
However, I don't think implementing something as complicated as having multiple top-level lieges is the solution here other than confusing the shit out of people. Several, also rightfully point out unlike in the the game, the Normans IRL don't get to shake free from their feudal obligations just because they also snagged England. People mention William the Conqueror several times here and its similarity to this situation.
It's strange gameplay wise, but yeah, there are instances of similar situations happening IRL.
