Like television does, with cuts back and forth on story beats.

"Okay, you guys set to work rooting around for the McGuffin in the library. Meanwhile! Group B, what are you doing?"

Secondarily, encourage (remote) communication if the setting allows it. Give one group a clue that will help the other group, but it must be conveyed.

Here's a trick I saw on a let's play a while back: have the players whose characters are not in the scene play NPCs.

Assign one group high, the other group low. Then roll a d20 and kill the high or low group that corresponds to the roll, then ignore those players while you deal with the others. /s

I used to play in a World of Darkness and later a GURPs game and both has upwards of 15 players. There was very little time that everyone was ever together. The ST ran little scenes with groups of us. Everyone else sat and watched the scene or split off into their own group and did some RP and then reported it back to the ST. The ST would send me to oversee RP he was worried would become too intense so I could act as a secondary ST (I also knew the WOD rules better than the ST did).

All in all it worked fairly well and everyone that played in the games knew the ST’s large format style and was either used to it or adapted to it.

If the split is going to be a longer term thing, I like to run 1 group at a time and have the players who are not in the split group run temporary characters or NPCs. Usually those are something like MCDM Followers/Companions or just simplified PC characters so that there isn't much of a learning curve, but it just depends on the people at the table.

If the party already has bunch of followers or NPC friends, it's really easy and people seem to enjoy taking the reins of their favorite NPC's for a few sessions. It's also a nice chance for players who like trying our different builds to have a small timeline to try something out with an NPC, and it adds the bonus of shared worldbuilding.

Once the first group is finished, we swap roles and pick up the second group.

Its best to keep this limited in scope, make sure its not more than a few sessions per group, and to only employ it occassionally.

However, if it's only for a part of a session, I go for the A/B storyline in a TV show strategy and tend to verbalize the "camera" a bit more, especially if it makes sense to give some subtle progress hints to the other group so they don't feel the need to worry too much about metagaming. If one group in in combat while the other isn't, I'll switch back to the non-combat group after every round or two. Gives everyone a little more time to get their bearings in a reduced party size and makes the combat action feel a little more intense with some good ol' tension and release.

"OK, as Jimothy unlocks the door and peers inside, it's dark and will take a moment for their eyes to adjust. Swords McGee, watching Jimothy's back, nothing seems out of the ordinary from the perception check, but he does see a flash of orange on the northern wall of the compound where your friends should be at, followed by the distant, unmistakable crack of your allies fireball spell."

"back to the rest of the group, Bobby Fireballs finished up last round by blowing up the guard station, top of initiative, the guard captain..."

It doesn't always go that smoothly, but you'd be surprised how easy it can be once you get in the rhythm of when to change cameras. Its also very important to briefly summarize a hook when changing cameras to transition everyone elsewhere.

If it makes more sense to stay with one party for the entire combat, I'll usually hand over a few monsters to the non-participant players so they have an opportunity to be doing something, if it makes sense.

As often in RPG, Which game do you play, what did you agree in session zero regarding the level of secrecy and the level of PvP. The answer is very different in a Vampire the Mascarade with an agreement on lethal PvP than on D&D full cooperative game

is there anything interesting happening while the party is split ? A shopping trip, especially for common items and with no pressure can be managed by the player without any GM intervention. On the other hand, having a PC telling the Prince about another PC behaviour requires some privacy.

Here are some stuff I use, depending on the game/context

• Small paper notes given directly to a PC (or given to the GM when being a PC) pretty great, especially when when PC have their SAN drifting low and you want to give different informations to everyone

• Limit the one by one time, like once, 10 minutes per session because if each players takes 30 minutes alone, sometimes twice, you end-up having PC playing 2 hours over a 6h sesssion, it sucks for everyone.

• Same technique as before, but keep everyone at the table, so players are aware of what's happening it also help saving time, in narrating what happened.

Then for the non active player, I tend to the rule-book on the PC side of the table, so they can use this time to review their abilities no matter whether we talk about PBTA like moves, D&D like spells or Cyberpunk-like gear. It's also the moment where they can take a small break to grab a coffee/snack or go to the restroom

"Don't split the party" never felt like an axiom to me, the more appropriate guideline is knowing when and for how long to shine a spotlight in player's affairs, but also to maintain a singular objective across the party in your particular narrative. A lot of my time playing (outside of combat) has been with a split party, and with my current Lancer campaign that I've been running for a few months the players are also comfortable trying things at their own pace.

It helps to have a Common Objective that all PCs share and want to fulfill, so the "main" missions of the narrative will involve all of them together, but any kind of bonding scenes, Downtime, narrative hangouts and such, don't necessarily need to have everyone together. You still should prod other characters about their thoughts on [X]'s PC idea/action/feelings, and give them a chance to intercede. Also when [X] talks about doing something, ask if they don't want to take someone together - or ask someone if they don't want to join.

Relationships between PCs will grow organically, so over time they'll have more interests and goals in common, giving you ammo for any time they want to do something. If two players have a shared specialty and one of them wants to act on that, ask the other what they think. If two players share a hobby, and one of them want to make a show of it, ask the other player for their opinion.