On this post Brendan talks about Persistent Campaign Settings. And cites the Majestic Wilderlands as one example he knows about (thanks for the shout out). In it he gives a number of observations and asks some questions.
Obviously, there could be some logistical complications with this. What if multiple groups are playing at the same time and affect each other? What if one group plays in "the past" with regard to other groups? It seems like temporal paradox could potentially be a problem, though realistically I don't think it would be difficult to avoid.The most number of groups I ever had running is two. I generally run the campaigns in geographically separate areas so there is little chance of overlap. In the past couple of years I had a campaign where my regular group was adventuring in the City of Viridistan and the game store campaign was taking place around the City-State of the Invincible Overlord. The two cities are several hundred miles apart in different cultural regions.
The closest to two campaigns interacting was in college, nearly 30 years ago, where my hometown player group ran into things that the college player group did and vice versa. The college group was of mostly of good alignment and the hometown group (which included Dwayne of Gamer's Closet and Tim of Gothridge Manor) was of lawful evil alignment. Dwayne was playing Lord Divolic a Lawful Evil Myrmidon of Set, and Tim was Count Travlin a kick ass fighter type.
The college group messed up one of Divolic's slaving operations, Divolic investigated and found out it was the college group. Finding out where they are he decided to send them the Bag of Holding that contained the demi-lich he captured in the Tomb of Horrors **. So the first game I ran for the college group after break, they had the Bag of Holding delivered to them. Unfortunately they had a paladin in the group who decided to Detect Evil and sense the bag contained a great evil. To the party sorrow, after all it was a bag of holding, they had the bag dispelled. Ironically Dwayne came to the same college a year later and everybody had a good laugh when he told them that he sent the bag and what was in it.
Stories, aside, the basic technique to running the two groups concurrently in the same campaign is to manipulate each campaign so that there little chance of the two interfering with each other. And if they do the one thing you have to avoid is a direct assault by one group on the other. Dwayne was gracious enough to realize that there was no way of going directly after the other group due to physical distance and came up with a fair alternative that worked out in the end. With the rise of virtual tabletop, and the internet the number of ways disparate groups can interact has greatly increased. Beyond the direct assault issue, just adjudicate what each group does fairly and don't play favorites.
Tomorrow Part 2
*I installed it myself although I needed a electrician's help in figuring out a weird electrical ground issue that wasn't from anything I did wrong. It was expensive but saved half the cost by doing it myself.
**Divolic and Travlin got to the main treasure room of the Tomb of Horrors and without touching or taking anything, Divolic held a bag of holding upside down above the demi-lich. Then Travlin grabbed some of the treasure causing the Demi-Lich to rise. Right into the Bag! Divolic shut it and that was the end of the demi-lich.