You're actually putting the assist in the wrong place. It goes on the Combat Hotkeys, not in the class-specific Key Map.

When you put it where you have it in a class-specific DPS Rotation, you're asking each guy to tell the others to assist him, even if you're not in that window. So the one you're playing is going to get told to assist all of your other guys.

Instead, put it right in Combat Hotkeys' DPS Rotation, above Do DPS Rotation in Virtual Combat -> all. Then the window you're playing tells everyone to assist, right before telling them to do their DPS rotations.