On the theRPGSite the RPG Pundit asks whether anybody runs two campaigns at the same time using a setting here.
I been running my Majestic Wilderlands for close to 30 years. A handful of times I ran two campaigns at once. The most was three while I was in college in the late 80s. The most recent was around 2003 when I introduced some friends from NERO LARP to GURPS Fantasy when I was still GMing my normal group.
I run my campaign in a naturalistic sandbox style. Generally I start out with what is the present of my campaign extrapolate a series of background events which forms the backdrop on which the player campaign. If what the players do effect these events I change them extrapolating from the changes that were made.
Things get interesting when more than one group is involved. This occurred a lot in college when I had a group at home and a group at college. There were times when both groups were wondering at the changes going on around them.
I generally don't set simultaneous campaigns in the same immediate geographical area. It minimizes the chance where a direct face to face meeting is called for.
The other technique I use is too minimize the overlap in their social group. For example two groups in City-State; one deals with the beggars and thieves another hangs around the Church of Mitra.
Still another technique is the slight time shift. A GM can throw in extra stuff to throw off time enough to make sure two groups don't end up in the same locale at the same time.
All of this done for the practical reason that is not easy to get a large group of people together. Aside from that I cheerfully let the separate groups run into the consequences of what the other group does. One group kills an important contact of another group, oh well that how it works out.
It is rare but sometime one group directly interact with another. Most of the time it just one or more member of the other group showing up for a session. I do have one interesting story.
One group in college revolved around a group with a Paladin of Mitra named Endless Star. Another a group at home with a Myrmidon (LE anti-paladin) of Set (played by Dwayne of Gamer's Closet). During the summer I continued the campaign at home with the Myrmidon. During the campaign they went through the Tomb of Horrors and managed to trap the demi-lich in a bag of holding.
The module stated that the demi-lich would rise if touched. Since it did not immediately attack and sank down after the players left the room they got an idea. With one holding the bag, Tim of Gothridge Manor, Dwayne touched it. As before the demi-lich rose into the bag and they quickly sealed it shut. Along with the demi-lich was all the treasure in the Tomb.
However this did not upset them because shortly before this incident was the infamous, "Rob read that again!" incident when they discovered that the doors were solid Mithril. With a ton of Mithril to haul out. they were not concerned with a few thousand in gold and gems.
After the adventure they were mulling what to do with the demi-lich and the bag of holding. Having heard of the college group and their paladin they thought this would be a good time to mess with them. So they hired a courier to send it to them.
So several weeks later and back in college everybody was ready to play again. During the first session up comes the courier with the Bag of Holding. Although I run my campaign as a sandbox this was unusual as it just totally out of the blue.
They cast Detect Magic, it was magic.
They case Identify, it was a bag of holding.
Now they were really scratching their head as a Bag of Holding was a really nice item to have. Out of the blue the paladin players goes "I detect Evil". And with the demi-lich in there the bag was evil as hell.
That cinched it for the players who had it destroyed. They figured it was a trap set by Bargle their arch-nemesis. (yes that Bargle) and never tried to figure out who sent it. They only learned of the bag content's a years later when Dwayne became a student at my college. While they were glad not to have dealt with the demi-lich, they groaned at the amount of treasure that was within.