When you do the assault on the control room go onto the first bridge were the glass breaks and clear all the enimies. After that go down bottom and the way you came one at a time jump off and youll land on a platform then the other person jumps and then if they die thell respawn on the platform then one at a time fall down and there will be another platform. Then the other person die and respawn were they are then youll be able to fall down and there will be two banshees were you are.