The trick to doing a multiple-GM campaign, which, I admit, I have only done a few times as I am very, very territorial, is a honest discussion at the beginning and to be able to compromise during the campaign.
You two need to sit down and discuss what things are off limits to the other and what things you need consensus for. For example, you could decide to run two parties who are pursuing the same artifact. Well this is two separate campaigns that overlap, but there needs to be coordination between the two GMs about the artifact, its protectors, common opposition, and the setting.
You also need to communicate as the campaign progresses. If you get in an argument you need to be able to resolve it. If the other GM is letting his party get to the artifact without any real opposition then you need to be able to tell him why this is unfair to you. He also needs to be able to listen to your concerns.
If you can have good communication and both of you are willing to compromise and work together then this can work and work very well, but if this is not likely and you are worried about conflict this will end in disaster.
I could give more specific counsel if you could share what your initial plan.
Hope this helps!
We do not as of yet have a specific plan, this arose from the fact that our gaming group is growing very large in size and scope and will encompass so many different types of roleplayers. The two of us are the usual GM's but we both have our big flaws, my friend doesn't do longevity, concrete worlds or plots and I can't handle sponetniety, erratic players or winging it.
That then is a recipe for disaster.
It sounds like you already have a split in the group between different play styles not surprising with 9 as that is a large number of players. One thing you could do is just run two campaigns in the same setting, but in different parts of the setting. Connected by setting, but separate. One of you run Thursday night and one of your run Sunday afternoon or something like that. Limit each to 5 or 6 players (or maybe you just limit yours if the other GM is fine with a large group) and make it clear to everyone this is just to give everyone what they want.
I suspect since you are asking about this you are 1) a little unhappy with some players, 2) a little unhappy as a GM with such a large group, 3) have a little disconnect with the other GM, but 4) you want to keep the group together as you are all friends. This is very reasonable and commendable in my opinion.
Now with two campaigns it can become a little competitive. Don't take this personally. I once had a player who started up his own campaign on a different night. I played in the campaign for awhile and I lost a few players in my campaign, but this is no big deal. In time things settled down with those who preferred his style of campaign in his game and those who preferred my style in my campaign. Choice is a good thing. In this situation I had a smaller group, but I knew all of them really wanted to be in my campaign, that is a nicer feeling then having a large group where half of them are there just because it is the only game in town, so to speak.
For example, you could both be adventuring parties working for a common benefactor, a church, king, demon prince, etc. You could try doing both parties working against one another, but that is more complicated. Plus with both of you on the same side you can easily switch players between campaigns.
Your campaign could be more story orientated and the other GM's could be more episodic.
Every now and then you can pull both parties together for a big adventure where they are all working together. This would be a fun roleplaying experience.
The key though is to develop a situation you are both happy with, but that probably shouldn't be that difficult.
Any suggestions on the technical side, like how to run the session while the parties are merged? How to trade off responsibilities? Both of us backseat GM for the other occasionally during games anyway, suddenly jumping in with random Metagame moments that aren't always unwelcome.
But what to do while the other GM is running the game? Well you might have a PC from one of the campaigns, but if not the other GM can run a NPC who is key to the adventure. For example, in Part #1 one GM runs the old sage who has been searching for the Big Plot Device for ages. This character is how you give information to the parties and as it is a GM this works well. Part #2 may begin with the old sage betraying the party and becoming a villain. In steps a new NPC who is an enemy of the old sage and brings in some more information, and this is run by the GM who isn't running Part #2.
You can also have the other GM help with running combat, especially if you have a large or epic battle to get through.
Another trick is that if the party divides up you can have the secondary GM take a small group to a side room to run. You could even build this into the adventure.
My personal opinion is that one GM has to be in charge during an adventure and the other GM is only there to support the primary GM. This has to be clear. If the adventure can be broken into two parts this is great as both of you get to run the adventure.
We do not as of yet have a specific plan, this arose from the fact that our gaming group is growing very large in size and scope and will encompass so many different types of roleplayers. The two of us are the usual GM's but we both have our big flaws, my friend doesn't do longevity, concrete worlds or plots and I can't handle sponetniety, erratic players or winging it.
Actually, this sounds like there's great potential, if you're both willing to be flexible.
One way to approach it might be a division of labor: one GM is primarily the background & rules guy, the other plays the NPCs. That's kind of how we do our superhero campaign: I'm basically the one steering the overall campaign, and I'm the rules guru; the other guy is the comix fan and gets more into playing in character and such. Of course, that's not a clean division: we work together on everything as much as we can. Before each session we meet for an hour or two to go over what's going on and what's about to happen.
It's worked well for us (we have 2 GMs and eight players, plus an occasional ninth). We've done a lot of party splits, and several times we've divvied up the bad guys so that neither of us has to keep track of everything. There are also several moments where the GMs need to break for a minute to confer (either openly or in the next room), but that's generally more a matter of checking for consistency than any real disagreement.
Another method is to swap roles as 'table GM' and 'NPC GM' - for this scene , you're running the NPCs and the other guy runs the overall scene; next scene the other way. Or each of you has their own special NPCs, and you always play that NPC whenever she shows up.
I've also done one campaign (wild martial arts - Feng Shui) where I and another guy swapped the GM role completely between each adventure. That way, we both got to play, and both got to GM. It was fascinating, in part because we never met to plan ahead, and never consulted with each other about plots, NPCs, and the like. It worked in large part because of the genre, but it was also fun because we got so we would set up cliffhangers at the end of our adventure as kind of a dare to the other to get the players out of it, or even explain how it happened in the first place ('you arrive home through the secret tunnel in the basement, step out the front door, and - uh oh! - you look up and there's a missile plunging toward you! ...Tim, will you be ready to run next time?...'). We knew the game was on when I had set up an NPC who was going to be important, only to have the other GM (not knowing my plans) assassinate the guy during the teaser.
The key to co-GMing is being flexible and making sure to communicate to whatever extent necessary to make the game work. Since you've been gaming together for a while, at least you should have a feel for how each of you interpret rules and such, so that should not be a big problem to work through. It can be a lot of fun (and it's amazing how effective NPCs can be in a fight when a GM is able to focus on their actions, rather than running the whole table at the same time).