It wasn't long ago that our lovable Dustin Burg encountered some problems gaining achievements while playing a co-op session of Crackdown. He was able to gain certain achievements, but achievements for removing gang leaders were not earned. Additionally, gang leaders he had killed during co-op reappeared during the single player campaign. We received an official comment from Microsoft regarding this issue. Basically, the company line goes like this: when players enter a co-op session, they are essentially entering the friend's version of Pacific City. Individual achievements can be gained during co-op, but gang related achievements cannot. This was done to prevent one player from doing all the work and essentially giving the achievements to the other player.
So, there you have it fanboys. If you want gang achievements, you'll have to make sure you're hosting. While this will no doubt irk poor Dustin (you should hear him rail in the latest Fancast) at least it explains the issue.
