It needs to be close range and it needs to be two final blow eliminations with no reload and no weapon switching in between elims. I don't know if MC is the best place for it since weapon damage is massively increased allowing you to get shotgun kills outside their normal ranges. I would suggest trying to do it in Control using a Light Weight archetype shotgun. I think the kill range on the light weights is at or within the close range requirement. If you're running around with a slug shotgun getting headshots or a shotgun like Felwinter's you are probably getting the kills outside the close range requirement even in regular Control.