Andy – You Asked.
POTENTIAL TK SPOILERS!!! CAUTION
There cannot be two caithdein, precisely. Sulfin Evend's state of affaris is a shattering precident.
Maenol is sanctioned caithdein for Tysan - that means, by Tysan's old charter law, he may speak for crown justice in the absence of the king or sanctioned crown prince. ONLY MAENOL can stand in for this responsibility.
Sulfin Evend swore the caithdein's oath - which is to the weal of the land - and holds the people subject to the land's prosperity. He swore this oath, which enables him the standing to ask for Fellowship intercession - he can ask for help as a caithdein might, if the land and the people who are in harmony with the terms of the compact are jeopardized. And legit, the Fellowship can answer.
This allows the Sorcerers, also, the opening to temper what happens to Lysaer - without that opening, the horrid little brush with necromancy would have had little opening at all - if not none, for Lysaer's survival… a Fellowship intercession could NOT have salvaged his life, while he was caught "under the influence."
The Fellowship agrees to stand on Sulfin Evend's oath in order to temper the impact of Lysaer, under the curse. It does NOT give Sulfin Evend any power to claim authority under Tysan's crown charter, and it certainly does NOT elevate Lysaer's status to crown heir in any shape or form. He would not, either, wear black, which signifies "shadow behind the throne" or perform the office of royal conscience for even a potential s'Ilessid heir.
Maenol's lineage is the legit Caithdein for the realm. No way Sulfin Evend would supplant him.