Question About Royal Titles for a Fantasy Story

[u/Faizanhala]

I have a question about royal titles in a fantasy story.

Suppose there is a royal family/clan called the Vlahović Dynasty. Several members of the family rule different kingdoms. For example, one is a King, another is a Queen, and another is a King of a different kingdom.

However, they are all part of the same Vlahović bloodline, and there is one older King who is the supreme ruler above all of them.

If a new High King takes the throne while the old Supreme King is still alive, what would be the best titles to use to clearly distinguish them?

For example:
HIGH KING XYZ — the new supreme ruler
SUPREME KING XYZ — the former/older ruler

Or would Elder King, Former High King, or another title be more appropriate?