You need a high light level and a full fire team of people you can talk to in order for this to be made easy for you. First off run through the mission with them and then when both of the brothers are out have the team focus fire on just one of them, then when his health is critical make sure they stop firing and you go to town with a shotgun on him (I advise you take out the red one first then the blue one, this is only because the red one will merge with the blue one so he's on a timer). If they do merge you will only get the blue kill so you have to run it again, make sure it's done before the weekly reset so you don't have to wait. Please reply if this makes no sense at all :D