1. Never chase the enemy into the RL room from the lower entrance. When your strategy is running in high gear, you should have one of your RL guys guarding the doorway at the lower bridge. Even when he is not there, there is usually someone shooting across the length of the bridge at the doorway. So absolutely never, unless in circumstances where you know your RL guy won't be there, chase the enemy through that doorway.
 
2. Always leave at least one of you guys on the quad platform. If your teammate leaves you to chase down an enemy, do not leave your post. You never want to leave the quad platform open, as your RL guys are depending on you that there will not be any (at least none that are of serious threat) enemies coming from the top area. Just having a quad guy sitting on the platform should effectively keep the enemies from being able to jump across.
 
3. Make a bind that says something to the effect of: "Coming in from lower bridge, do not shoot!!"
(In the console, type: bind <key> "say_team Friend coming in from lower bridge!!!")
This bind is extremely important, as the quad guys will go through that area quite often, and I'm sure they would not appreciate being shot by their own teammates.

Next: DM2 Strategy Guide