Proceed directly and normally as you would in Normal Campaign, until you are at the part where you must destroy the Anti-Aircraft Battery. Walk away from the gun and towards the large canyon. Go to the part of the ledge closest to the other side, and grenade jump (Crouch jumping also works in the right spot). If done correctly, you will survive to make it to the other side. However, the second player who was left behind will be unable to make this jump for an unknown reason. Have them jump into the chasm and die. They will respawn on the other side.
On the side, walk around at your leisure. Some things are nonexistent as physical objects - you can walk through certain fences and a large boiler. Invisible walls appear in certain areas. The nonexistent objects are that way because they were programmed to just be seen, not to be interacted with.
Note: Do not throw grenades while exploring this area, as it will abort the glitch entirely.
Any grenades thrown will (for some reason) create a very short height barrier that sometime causes death if touched. When you are done exploring, have one player commit suicide to gain grenades, and grenade jump back to the real side. Have the second player commit suicide, and you may then continue normally.