How do you perform/setup a Co-Op Execution in Ryse?

The amount of times I've accidentally performed Co-Op Execution is staggering. I know it will be performed when I see two swords crossing a skull above the enemy's head. The thing is, is this random or is there a way I can force this to occur?


Solution 1:

I find that it only happens when you are close to your ally, especially when you are in front of your enemy and your ally behind him.