So me and my arena partners were playing 3v3 last night. So we noticed that if (lets say your team and PR ratings are 1600) 2 of your team mates are dead and the last one is alive that if he leaves the arena match before he dies both of you still in the game dead are awarded the achievement "Just the Two of Us: 1550"
This works for all 2v2 achievements like 1750, 2000, and 2200.
If you need any more clarification please just tell me!