You'll need to setup a game with both Elanor leaders (France and England) in the same game by enabling duplicate leaders/civilizations in the options. To have the most control over the situation I setup a duel between the two and went for a domination victory.
I focused on taking out the high population cities first, including the capital for the victory, to force a lot of pressure on a smaller, singled-out city. I dropped a few spies in the target city to continually keep removing the governor and create unrest to ensure loyalty kept dropping. After about 20-30 turns of dedicated effort I was able to get it to flip. I started in a later era so I would have earlier access to spies and turned off the turn limit in case it went well beyond the actual victory.