Just have a fire team of 2 you and one friend and the other 2. Play the same missions at the same time. While using party chat. And also you can alternate teams so you don't have to pay with the same guy the whole time. And if you need 3 to beat a mission you can just let a random join or 3 of you play the mission and win then 2 of you can reply the mission with the other guy. Then just get back in teams of 2 and stay on party chat. You can always alternate who gets left out