Mixing succession laws- is there a downside to this?

After much diplomacy, war and diplomatic wars I've reunited France and Aquitaine. All my land is in those kingdoms.

France has Primogeniture sucession. Aquitaine has Gavelkind. I personally control the Duchy of Normandy- most of my demesne holdings are there.

I set Aquitaine as my primary title. This means inheritance works as follows:

• All titles in Aquitaine are divided among my heirs. In practice, this means my first child receives the Kingdom of Aquitaine. That's it because there aren't any more titles.
• All titles in France are Primogenitured to my eldest child.

This appears to give me all the bonuses of Gavelkind except without dividing my lands on death- it might be too good to be true. Aside from this being a bit cheesy, is there a downside to this I'm missing?

Both Kingdoms will definitely pass to your eldest. All titles in France will pass to your eldest along with the Kingdom. If you have any land outside de jure France, I think it will be effected by Aquitaine's Gavelkind, and given the amount of inheritance going with France, that will mean your other children inheriting some land.

Downsides: Obviously if you get any land outside de jure France you risk losing it on succession.

Under Gavelkind you may only grant your eldest a single County. I often gift my heir with multiple counties/duchies to help him gain prestige, and to cut back short reign penalties with at least some of his future vassals. It's also useful to have him hold excess land gained from larger scale wars, because you will end up getting it back. Under Gavelkind, most of these options are off the table.