Another question:
If a duke who already has a liege, inherits a duchy in another country, he will stay under his current liege, and bring the newly inherited duchy into the fold of his original kingdom, I know that.
BUT: what happens if said duke isn't actually the ruler of the duchy, just the husband of the duchess? It's clearly possible for a couple to have equal level titles in different kingdoms, so maybe the newly inherited duchy won't merge into the other kingdom.
The tooltip just says the usual "the title might leave your realm" stuff.