Here is what worked for me, finally!
From the main multiplayer menu press the Y button to invite partner, and select a friend. There will be a box in the top middle of the screen showing your partner.
Now select game browser and select a game (Not Extraction) that has only one to two players. You need to find a match that has not started yet. Once you've joined ready up and when the match starts achievement should pop.
It took me several tries to finally get it to work. It did not seem to work if joining a game in progress or when joining extraction matches.
Hope this helps.