You can get the achievement in Co-op, but it will be harder. It uses individual score in either scoring method, not combined score. So you and your friend could get 16000 points on the first level, but if you each only have 8000 you will not get the achievement. My friend got this on Co-op last night by turning on the Thunderstorm and Mythic skulls, Legendary difficulty. He got just over 15000, and the achievement.
I found that doing the levels on Heroic with Thunderstorm and Mythic work pretty well. You still get enough points, and it's easier than trying to do it on Legendary.