If no equal titles are available they will be given lower ones. For example, if you are the king of two countries, the second oldest son will take the second kingdom on ruler death. If you have multiple children eligible for inheritance, the others will be given titles starting with those of the same rank as your primary. By default this means that your eldest son will inherit the Primary Title along with the Realm Capital and any direct De Jure titles associated with it.
At the start of the game, and well into the first few centuries, you will only really have access to gravelkind succession which divides your lands amongst your eligible children.
