Definitely Yugi and Percy. Yugi is the heart of the Yugioh series with his connection to the Millennium Puzzle and his dueling skills. Percy, on the other hand, is the hero of the Percy Jackson series, a demigod with his own set of powers. They would be the ones to bring the two worlds together and lead their respective groups in any adventure or conflict.
Yugi, Percy, Joey Wheeler, and Annabeth Chase could all be main characters. Joey is Yugi's friend and a great duelist in his own right. Annabeth is Percy's intelligent and strategic friend. They could add different dynamics to the story. Yugi and Percy would be at the forefront, but Joey and Annabeth would be there to support, offer different perspectives, and get involved in the action. For example, Joey might get into a friendly rivalry with some of the demigods, while Annabeth could use her wisdom to figure out how the Yugioh cards' magic works in relation to their world.