Co-DM's are great!
I love them. Let me explain how I've used them in my campaign.
My characters started out in Sunless Citadel and then jumped into RttToEE. By the time they got to the Air Temple, they really hadn't been challenged in a while. So, I invited person A to Co-DM an encounter with me. Basically, when the PC's assaulted the Air Temple, a third faction was going to T-Port in, try to steal something from the Air Temple, and mash the PC's a bit, if necessary. Sort of a three way dance.
To make the surprise better, we floated the story that he was thinking of joining our game and played the Wizard's Dwarven Cohort. At round 5 of combat, he starts interrupting me with "He cannot get to that square." Or, "While moving forward, that Orc is cut in half, from seemingly nothing." At round 7, His DM screen goes up, and he starts placing mini's on the board.
Everyone freaks. I play dumb for a bit, and have my bad guys attack his, and watch as mine get wasted, then I leave the room saying, "I'm going to roll up some new characters." and let him beat on the PC's for a bit. Eventually his group T-Ports out, I come back in and resume DMing.
Really Cool.
The second time I needed help for a huge battle, was at the end of the RttToEE, where I had all the cult members summon Imix. I had my DM from another game, and his wife play Falrinth and Smigmal, since he plays casters very differently from me, and she plays a mean Rogue. They basically got to play PC's against my PC's. Worked very well. Falrinth and Smigmal's players had rules of engagement, and knew that they weren't the stars, but were there to try to add some variety and were very gracious.
The third time I needed a Co-DM was my last session. The players had run roughshot through City of the Spider Queen, and were now back to the original meta plot for Bastion of Broken Souls (Set up 4 years ago, during Sunless Citadel with the plague of soulless births), I upconverted the Titan to a Colossal sized Kraken with the Epic Pseudonatural template added. After the building the beastie with 700+ HP and an 11 attacks of +75 and his peons I was worried that I would go soft on my players, and not utilize all of the creatures bells and whistles. My party Druid has over 300 hp. This Epic stuff is not in my wheel house. My Epic Mystic Theurge just picked up Automatic Quicken Spell, asking if it works on both spell lists....
So I recruited another Co-DM to run the Big Guy, with instructions to kill as many PC's as possible, take no prisoners. And ran about 5-6 Peons myself. Worked very well. I felt in control of an encounter for the first time in months.
As I build out some insane encounters to upconvert Bastion of Broken Souls last act about 6 to 7 levels. I'm going to need that Co-DM to run all this stuff. Our next encounter will have SO MANY Demons. When you are dealing with Orcs 1-18 you can wing it a little, but when the retainers of Demon Lords all have 15+ HD and centuries of combat experience, it helps to have a second set of dice on my side.