Running a campaign with two different and unconnected parties is interesting. I have a full and complete view of the world,* but the two parties form a lopsided Venn diagram of experience.
Team Forest has a tendency to wander to and fro in PlotLand and run into plot in the form of not-so-random encounters. Or, when that doesn't work, they head over to their hook NPC to ask for plot. As a result, they know a small amount about a great many things. Their focus tends to be "Where?"
Team City, on the other hand, spends most of their time rolling Gather Information or misc. research rolls and their head researcher occasionally requests to skip all random encounters. (The requests don't work, but they've also been staying out of the tall Plot grass for the most part.) As a result, they know fewer things and are a lower level, but have a far greater depth of knowledge on a few topics. Their focus tends to be "Why?"
This dichotomy is particularly clear regarding the Two Obviously Evil NPCs who've popped up now and again to cause trouble. Team Forest has met both, knows a bit about both, and kinda hates both. Team City knows a TON about one of these NPCs and has apparently never even met the other. (I didn't realize this, somehow, but in retrospect it makes sense given what they've been up to.)
Meanwhile one party is trying to find the other party's hook NPC (they have different ones, but each party is distantly aware of both), which would normally be easy but currently isn't, and small events keep domino-effecting from one party to the other. As we approach closer to the meat of the campaign, I hope this interaction only increases, as I find it entertaining. I also suspect that over the next couple sessions certain large, overarching plot things will become clearer to both groups, (assuming they don't all die), which should be a good transition for the next phase of the campaign.
One thing that I've been struggling with in each party is the distribution of information and plot. When a character comes to me and says "I'm going to try this!" even if it's something I never expected, I try to say yes and give them some interesting result or information. This accidentally killed one of my NPCs once, which was fine and led to interesting changes. I dumped a TON of backstory when some of my players went all detective on me, interviewing witnesses, digging into old history books, etc. Another NPC/subplot got rescued and immediately sent packing, so that undiscovered story is still lurking out in the woods somewhere. (Yet another subplot faded quietly into the background because it caused an aggressive and unfun amount of party conflict.) That said, I occasionally see frustration among my players when their hooks are as lost as they are or doing a research roll at a library doesn't lay out the plot neatly, or when different sources have slightly askew information. (Trouble of getting from books rather than people, TBQH.) And I know both groups have occasionally felt like they weren't sure what their next steps should be. On the one hand I want to give the players what they want, on the other hand "player rolls a die, DM sits and tells a story for the rest of session" is...really fucking boring, even for me as the DM, and inorganic/OOC hints are awful, particularly for what is nominally a horror-adventure game, which implies a level of uncertainty. (Neither group has ever actually been stuck with no idea what to do, fortunately, so this is hopefully not too big of an issue, but it's something I'm watching, particularly given the VERY different player approaches to seeking out information/plot, as well as the different player definitions for what constitutes information/plot, even within parties.)
That's more accurately broken down into two concerns: 1. Balancing overall quantity of plot with appropriate pacing. 2. Catering to different styles/preferences within the same campaign.
Another complication that hasn't yet been an issue, but that I'm watching out for, is that there are some tasks/scenes that are slated for one party and others that are slated for the other. I could make those combined sessions instead, and may yet. I dislike applying plot armor, but for certain things I feel it's necessary.
On a sillier note, the last two sessions introduced two new NPCs, one to each party. These NPCs may never meet, are even less likely to meet on camera, as it were, and it could be that neither party ever meets both, but I totally ship them and this is ridiculous.
* More than my players, at any rate. The fact that I make up most sessions on the fly notwithstanding.