Play the mission alone. After exiting the third tower and killing all the Flood, there should be a group of Marines waiting in Warthogs and Tanks. The Arbiter willenter a Phantom and start to leave. Get in the Scorpion and destroy the Elite Phantom. The Arbiter will survive and fall on the slope below. Get in a Warthog and have the Arbiter jump in the back, then proceed as usual. The Arbiter will accompany you even though he normally is not there. After destroying the two Scarabs, the same Phantom you destroyed earlier should spawn another Arbiter on the ledge where he normally calls for you. The original Arbiter should still be in your Warthog. Drive the Warthog on the side of the ledge until you reach where the new Arbiter is standing. The Monitor will activate a light bridge and both Arbiters will now enter your Warthog. You can now drive both of them around. Note: After the cinematic with the Prophet and Johnson, only one Arbiter will be there.
When the Prophet of Truth begins to say "My faithful, stand firm...", immediately kill the Arbiter so he falls to the ground. This easily done by using a weapon that can knock him down with one shot when his shield is depleted, such as a pistol, carbine, BR, etc. Be precise; if you do not kill him you must start the rally point over. If done correctly, the Arbiter will not have time to get up before the cinematic. During the cinematic you should see one Arbiter with a sword standing still if you killed him close to the elevator while another Arbiter takes part in the cinematic running forward. Continue throughout the level with one Arbiter until you reach the beam bridge that allows you to cross to the crashed Pelican. As soon as you reach this section, another Arbiter will appear and you will have a second Arbiter. This glitch can be done on any difficulty setting in single player mode.