What would you put in this dungeon?

[Endril_69]

I'm running an in-person game and a discord game. It's the same campaign world, and the parties are similar size/level/wealth. I was thinking of running them through the same dungeon. I might use magic to prevent them from running into each other, like they're in different planes, but can do things that will effect the other party. Like if party one kills a certain monster, it opens a door for party two. Or maybe party two has to find and disarm a trap for party one. In addition to any logistic advice you might give me, what kind of challenges can you think of that would require them to work together?

[Fizban]

Well first question is whether you're going to try and do this simultaneously, since that's the only way to have active interaction between both parties and for the question of them running into each other to be really relevant. The easiest method is just that whichever party goes through the dungeon first goes first, and the next party comes along some time after which is long enough for some monsters to "restock" and changes to happen, but close enough that major changes remain.

Thus, you'll need some type of scenario where it makes sense for there to be new foes even after the previous team cleared the place out, such as fiat-tier planar gates or somesuch that supply a stream of monsters which can't be stopped by the first party.

Unless this is supposed to take several sessions, in which case yeah having them (stuck) on two different planes would probably be best. For standard planes you have the etherial, but that allows active sight and certain interactions: you can take simple published inspiration from force effects and such that specifically work on both planes, but you've got no explanation to keep them from crossing paths. The Shadow plane on the other hand, does not allow active, or really any interaction, aside from a couple very specific spells. IIRC the Shadow plane is supposed to react somewhat slowly to major material changes, sometimes not at all, but you can simply override that for this particularly unstable zone and have walls or even doors opened on the material affect the Shadow.

There's not really any ways for the Shadow to affect the real, but there is the distance factor. IIRC, while all movement through the Shadow plane does not necessarily function at the speed of the Shadow Walk spell, I think it is supposed to be faster. Thus, you could have the Material team wandering a much larger area and thus going much slower, while the Shadow team has shorter distances. The shadow team might be blocked by walls or doors the material team ends up breaking, while the shadow team gets to the "endpoints" faster, popping through gates into rooms where they can do things that affect other parts of the complex. And of course, the Shadow plane can leak monsters just as well as any other.

[Particle_Man]

Maybe a temporal loop dungeon, so that each party is in the past of the other (both relatively near and far, because loops)? They might even be able to leave some messages for each other, but sometimes the messages get destroyed, partially or wholly, by time.

[Inevitability]

The dungeon is a complex structure, built across more than one plane, that forms a sort of multiversal orrery (https://en.wikipedia.org/wiki/Orrery). The dungeon is defined by nodes, which are aligned with Good/Evil/Law/Chaos/Fire/Earth/Water/Air: nodes contain monsters typical for their subtype.

The twist is that any two opposing nodes are in different halves of the dungeon: one party might have access to Good, Chaos, Fire, and Air, while the other has access to Evil, Law, Water, and Earth. This takes the shape of a hub room with the entrance, and four branching paths to the accessible nodes.

Strengthening a node changes conditions throughout the entire dungeon (both halves): if one group of PCs heads into the Earth node and slay its elemental guardians, then gravity might weaken throughout the entire structure, or the floor might become brittle and prone to collapse, or the Air node might suddenly swell in power and send a few genies to secure the other team's hub room. The thing is: if the other team fights the Air node, all they'll accomplish is restoring balance to the overall structure (obviously, have new Earth/Air themed monsters and obstacles ready when that happens, so the rooms feel fresh even if they have the same theme).

Only when each alignment and elemental pair has been 'unbalanced' (which will require coordination: put some kind of communication device at the hub?), the orrery destabilizes and falls apart, leaving the two parties standing unharmed among its incredibly valuable debris. Also, maybe some villain was going to use it for an evil ritual and can't do that anymore, idk.

[Halrax]

Matters a lot on party level, since that changes what measures need to be put into place to keep them from running into each other. For lower level parties, having a simple dungeon faction/roster system (or even just only one faction like a huge tribe of goblins) that parties enter from opposite sides and defeat in a pincer movement could be interesting.

Also decide whether the parties are ever allowed to meet each other, even for the climactic battle. If not, you could create two separate corresponding dungeons with a similar room layout, but an action in one room influences the corresponding room on the other side. Or each party has its own gravity (opposite to each other) with lots of pits that connect to each other, so a monster killed by bull rushing into a pit (or just a collapsed floor) falls into the other dungeon (or into a shared bottom of pit space).

But by far the simplest solution is having one party go through the dungeon, restocking the dungeon, and then the other goes through, though that only works if the number of sessions per run-through is low (so you can finish the 1st runthrough before starting the 2nd). But that's clearly not what you were going for.

To get back to basics, what is each party's goal in the dungeon, how will their actions be in service to that goal, and what consequences from the other party's action will influence their ability to achieve that goal (or even what the goal is)? How big is the dungeon (in terms of number of sessions to accomplish goal)? Can you count on your players being willing to solve half of a puzzle while having to wait for the other party to solve the other half?

If the parties were able to interact with each other or if one of the parties were NPCs, then the interesting thing to do is for the parties to have different and incompatible goals. Since players set their own goals, you can engineer a way for the goals to come into conflict, or just let them both win.

If you're going to keep the two parties in pace, you need a way for the timing to line up as much as possible, and for any effects from one session to be delayed until the next session. So if party A does action X on Day 1 in session 1A, then party B does action Y on Day 1 in session 1B, the results of action Y can't be felt for party A until Day 2 in session 2A because you didn't know party B was going to do action Y when you were running session 1A. And logically it should work the other way as well.

Actual ideas (even though everything is dependent on specific factors): A giant hourglass with a red side and blue side. Party A wants to keep the red side from emptying, so they take actions to change it so that the blue side's on top. Party B wants keep the blue side from emptying, so they take actions to change it so that the red side's on top. But they don't know what way the hourglass is facing or how much has fallen until the start of the next day/session, and you make sure to keep sessions to one day of in-game time.

Then you provide each party with a win condition to complete before their half of the hourglass runs out. Now each party has a race against time, with the choice between accomplishing their goals or buying more time to do so, and they don't know how much time they have left. These dungeons don't even have to be connected (in fact they shouldn't be if you don't want the parties to run into each other) but each party has both their main objective and a secondary objective: reach locations and/or accomplish tasks that can turn the hourglass to their side, with each task only working once.

You can tailor the challenges to each party, and after one party succeeds have the hourglass be occasionally or permanently turned in their favor. And you can check the times that the hourglass was flipped, and then tell each party at the start of their next session which way the hourglass is flipped and how much sand is in each side.

And of course you can also include other connections between the dungeon or dungeons, but any changes have to be delayed a session for causality reasons. And it's a lot either to explain away one delayed factor than it is other factors. Though traps, secret doors, locked doors, and other dungeon changes can still be triggered by solving puzzles, casting a specific spell or other effect at a particular feature, or other interaction bits. But I don't think it's possible for the dungeons to connect unless you're capable of joining the parties on a whim (which presumably you aren't), so dimensional anomaly/magically linked dungeons/some other separation it is.