At the beginning of the co-op campaign, there are two separate parts to play. You will automatically get put into "Prologue A". If you're the first or the third person to join the room. If you're the second or the fourth, you will play "Prologue B". Either way this achievement will pop after only a few minutes of playing. The Prologues are complete after defeating a giant akrid spider. If you want both achievements, just restart the campaign and switch places with a friend. It takes about 4-5 minutes to complete the prologue and difficulty does not matter.