Army of Two (EU)

Afghanistan has the most. Once you parachute in, the back to back mode will contain at least 4, and possibly 5, 6 or 7. The first 2 come from your starting viewpoint, then turn around, take out the next 2 (try to do it fast to stop them blowing the other one up), and then watch the directions to get any more.

Once the back to back is over, 1 or 2 more may come from the direction of the 2nd lot from back to back (ie from the cave area).

Further on there are a few more inside the mission, specifically 2 who come out after you kill the mounted machine gun you need to Step Jump over the boxes to get behind.

Mission: Destroy Bunker in Iraq Base. Cross the bridge, in co-op snipe mode kill 2 soldiers, quickly move ahead and man the turret. Waste some more enemies, including some martyrs.
After 5-6 restart checkpoint you will receive 2 achievement: "Two Eyes are Better Than One" and "Fission Mailed"

What I did was this. Go to afganistan, and after the parachuting part where you take out the helicopter you will go back to back with your partner. While you are back to back at least 10 martyrs will come and if you kill all of them it should only take you around 10 minutes. Just keep restarting once the back to back is over.

The best way to do this is at the beginning of level 2. When you go into back to back, focus on the area ahead with the box on either side. Enemies will run towards you from the two paths leading away with bombs attached to them. Shoot the bombs to kill them and accumulate kills towards this achievement.

You will know you have done it correctly because they will explode.

There is not enough martyrs for this achievement in the play through let alone this level, so after they all come out (about 6 of them) rinse and repeat.