Anyone ever run multiple adventure paths/modules etc, concurrently?


After quite a long hiatus I finally got to begin a campaign with a group, and I'm running the Princes of Apocalypse campaign for 5e. So far my group seems to really like it and I love "4 element" genre of fantasy. Anyway, that isn't the purpose of the topic. I've been playing for about... 16 (holy crap has it actually been that long??) years now. I have always wondered, while reading fantasy books, playing video games, or tabletop games, why there's usually only ever 1 main storyline with a big bad evil guy. After thinking about it for a long while, there's no real reason stories should be limited like that. So with that in mind, I've always wanted to run multiple major story lines simultaneously and just see how the adventurers decided to weigh the consequences. Clearly the end result is going to have HUGE implications on the world due to whatever the group chooses to confront. I think this would be a fantastic way to apply pressure to the party from multiple BBEGs to see how they deal with the realization that their struggle is probably not going to save the world from being dominated by evil in some form regardless of which story(s) they try to take on.

Clearly it would take a very skilled DM to keep up with all of the information, but I just can't help but wonder what the party would do when confronted with War of the Lance, Princes of the Apocalypse, and Rise of the Runelords (clearly these stories would need to be retrofitted into whatever world the characters are in). I just think it would be fascinating to experience and I was wondering if anyone has ever done this. If so, do you have any good tales of their inevitable demise?"
Never on that scale, but I have run adventures where there have been different events occurring at once, and the path of the campaign was shaped by both what the PCs do and what they don't do.

I think the biggest hurdle would be to maintain the sense of PC agency, so that it feels like they're actions continue to have an impact. Too many threads and it could easily feel like events are unfolding regardless of what they do.
This is an old thread, so I hope someone gets a notification for this post:

I am currently running 2 campaigns, set in the same home brew world, at the same time (in game and RL). The first has been running for almost a year, the second I started about 2 months ago. The parties are made up of different people, with some overlap. Any overlapping players have separate character sheets for each campaign and are asked to not metagame too hard on what they already know about the other parts of the world. Don't get caught up when the metagaming does happen, because metagaming happens. I usually just remind them to play the character in front of them, not the one at home.
Pros: It has forced me to do more world-building, to tighten up motives and factions, and to flesh out parts of the world that would probably never be explored if there was only 1 campaign. This means that in the future I will have more encounters already created, different campaign paths available, and a solid, consistent flow to the storytelling beyond anything I have done before.
Cons: I have to track a lot of stuff, mostly time passing and all of the relationships that are happening. Making sure that I have multiple region specific histories, artifacts, magic items, and demographics that are still related to each other is also a bit challenging. My pantheon structure is strongly influenced by the book American Gods, so tracking influence from each party and applying it to both is important.

My Advice for someone else that wants to try this:
-Plan on lots of planning. Retroactively adjusting things can work (I have had to do it a few times for minor details) but having a flowchart for all concurrent campaigns is a must, especially if they take place in the same region.
-Create an "Influence" Tracker and use it religiously. Know when a party disrupts a slave trade, or makes a duke happy by saving his daughter. At early levels it won't matter too much if the campaigns are in different regions since the PCs are only affecting small villages and communities. At higher levels though, when their decisions might shape kingdoms or factions, you will want to know where they stand should their paths ever cross, either physically or by reputation.
-If you are using a home brew world (I wouldn't recommend it to start out with) make sure it is rock solid.
-Know up front if you are okay with the 2 parties meeting and what that might look like. It could be a group of NPC adventurers that cross paths, or maybe a huge session with both groups where they team up or fight each other. Make sure the players know what you decide on and the potential for good and bad things to happen.

My groups know that that each party can and probably will influence the world enough to affect the other, and that if they both make the right decisions, will end up meeting at some point. Judging how my second group plays, it probably will not be a peaceful encounter, but I already have a big medieval feast dinner party planned for that session. Because it will definitely be epic enough to warrant an event like a medieval feast.

Happy Gaming