Vaarna_Aarne
Notorious Internet Vandal
IIRC all the counties and duchies will be split among the children, but the heir will be the one who gets the empire/kingdom/duchy title and the capital demesne at minimum, the rest become his vassals. However, the junior heirs will get claims on the sibling liege's titles.
Though if you have multiple kingdom titles but no empire title, split happens. Same if you have multiple empire titles or duchy titles (without kingdom title of course).
Siblings can also be subject to conga line vassalage, since the counties are also split by de jure duchy, so the sibling who gets the duchy is the lord of the siblings who may have gotten a county in that duchy.
Though if you have multiple kingdom titles but no empire title, split happens. Same if you have multiple empire titles or duchy titles (without kingdom title of course).
Siblings can also be subject to conga line vassalage, since the counties are also split by de jure duchy, so the sibling who gets the duchy is the lord of the siblings who may have gotten a county in that duchy.