How to protect titles during succession?

Playing as Duke of Normandy and I conquered Brittany. I got the tooltip that said I would lose that title and a few smaller ones in a succession. I changed my laws to Elective Gavelkind but it still says I'm going to lose those titles.

My oldest son and heir has the title 'Heir to the Duchy of Normandy' and the son after him is 'Heir to the Duchy of Brittany'. If my character died, would Brittany become indepedent? Is there any way I can form Brittany and Normandy into a large kingdom somehow so it's all one landmass? I tried creating the 'kingdom of brittany' in one of the menus but I need 200 Piety and only have 170, so not sure if that would solve my issues.