MySQL: unique informations cross 2 tables

I have 2 tables battles & battle_user structure: battles table: id create_date 1 2015/... battle_user table: id battle_id user_id 1 1 1 2 1 2 Only 2 users accepted in each battle, the question is: Is there a way using primary/foreign keys (or indexing) to prevent insert another battle for same 2 users? prevent this: battles table: id create_date 1 2015/... 2 2015/... battle_user table: id battle_id user_id 1 1 1 2 1 2 3 2 1 4 2 2 prevent create 2 battles between same 2 users