Duke in different Kingdoms?

So here's the thing, through clever marriages I've made myself Duke of 3 different Duchies. They are currently all under the rule of the King of Lotharingia, but as soon as he dies the Kingdom will split into different Kingdoms (Gavelkind succesion). The Problem is that each of my Duchies are located in those different Kingdoms, namely Bavaria, Italy and Lotharingia. I can't be vassal of multiple Kings at once, as far as I know. What will happen to me once my ruler dies?

Solution 1:

De Jure tryumphs.

If a King holds several Kingdom titles, and there's a Duke with a Duchy in each Kingdom, should there be a split, the Duke will be part of the Kingdom that is his de jure liege. His de jure liege is the Kingdom that his main Duchy is in.

For example, let's say that King Jaques has the Kingdom of Aquitaine and the Kingdom of France. His vassal Duke Pierre has the Duchy of Aquitaine (De Jure Occitan) and the Duchy of Poitou (De Jure French). If Pierre's main title is the Duchy of Aquitaine, if Jaques dies (and the two kingdoms split), Pierre will be a vassal of the Kingdom of Aquitaine (And the Duchy of Poitou will be a part of the Kingdom of Aquitaine)