See if two table rows exist, inner joined to another row

Imagine I've got a sport where I've got a table, MATCH, and a table PLAYER, and the table MATCH_PLAYER. The MATCH_PLAYER table obviously links a certain match to certain players, and in the table MATCH_PLAYER I store information such as the position played, number of points scored etc. by each player. The MATCH table contains which team won, what date it was played etc.

I can make queries such as "what is the % win rate when player_id plays" using this, when you supply the player ID:

    SELECT
                    sum(case when m.win_left = mp.left then 1 else 0 end) * 100.00
                        / count(*) as win_percentage
    FROM
                    match m
                    inner join match_player mp
                        on mp.match_id = m.id
    WHERE mp.player_id = [whatever]

However, what I don't know is how to say "what is the % win rate when player_id x and player_id y play together".

This is obviously a fundamental idea behind databases but I can't for the life of me figure out the terminology for what I want to do, and so searching for it has been hard.