Creo que puedes usar case when then
sintaxis de la siguiente manera:
SELECT CASE WHEN player1id = 1 THEN player2id ELSE player1id END
FROM players WHERE player1id =1 OR player2id=1;
Alternativamente:
SELECT CASE WHEN player1id = 1 THEN player2id WHEN player2id =1 THEN player1id END
FROM players WHERE player1id =1 OR player2id=1;