I’m running two groups in the same campaign region. At least, I try to when they don’t cancel! So basically the two groups share the same Campaign Wiki. That means that the first group can sell a treasure map at the inn and the second group will be able to buy it for a price at the same inn. A hireling can be found at the inn talking about the adventures he had with the other group. Stuff like that.
This necessitates a certain amount of synching between the two parties.
I’m not using the Table of Despair. That would make the synchronization very easy: Every session is a trip to a dungeon. Those who don’t make it back out by the end of the session must roll on the table—and despair.
Worse, I use the Death and Dismemberment table. That means I sometimes have characters with broken bones that need to rest for many weeks. When the players return to the table and feel like playing these characters again, this assumes a big gaps in the time line.
The solution I’m using right now is that the party sitting down at the table implicitly determines how much time has passed.
I generally use weeks as a unit: Travel to a dungeon, explore for a day or two, return to base: it takes a week.
Here’s an example of how it works:
I just need to remember to announce the jump in time at the beginning of the session and ask: “So, what has your character been up to all this time?”
Alternatively, I ask them via email. I create a Google Group for every campaign. Sometimes players will post their plans for the next few games in-game.
This has worked for me so far, but I don’t have any long term experience.