i got both guys up there (1 on each side of turret) by just telling them to go to the left door only. i did it very fast:
1. took cover on left pillar
2. peeked around left side; looked at left door on 2nd floor.
3. told teammates to stack up on the left door on 2nd floor.
4. QUICKLY threw grenade at door behind bomb on bottom floor.
5. BEFORE GRENADE EXPLODED, told team to disarm the bomb.

voila...both guys are up top.

AS FOR THE CRASH, THATS CAUSED BY THE BODIES PILING UP. TO FIX THIS, STAND FAR AWAY FROM THE TURRET (BEHIND THE BOMB), AND DO NOT LOOK AT THE BODIES. THEY WILL GO AWAY IF YOURE NOT LOOKING AT THEM/ STANDING NEAR THEM.
PUT A RUBBERBAND ON YOUR CONTROLLER (STICK 2), SO YOU'LL SPIN IN CIRCLES WHILE LOOKING AT THE GROUND. THATLL PREVENT THE XBOX FROM TURNING OFF FOR INACTIVITY.
HOPE THIS IS HELPFUL