You must have the Main element on the elemental grid as Dark and atleast one Light minor element on the grid. Also you cannot have any ally with an element which matches the minor or major elements on the grid.
Example of non working set up.
Minor= Light, Light, Fire, Fire, Water
In your party: Emil, Marta, Genis, Bullfrog
This will not work because Bullfrog will become involved in the attack and ruin the Co Op Attack.
Example of Working set up
Major element = Dark
Minor Element = Light, Light, Dark, Dark, Wind
Party: Emil, Marta, Bullfrog, Wolf
This will work because neither the bullfrog or wolf will get involved and Marta and Emil can do the special.