The advice you usually hear is to stop the party from splitting. I think that is backwards. Splitting is a sign the table believes the world is bigger than the corridor you put them in. The job is to handle it without three people going quiet for forty minutes.
Cut on questions, not on time
The instinct is to give each group equal minutes. Do not. Cut away the moment a group asks a question that cannot be answered immediately, and cut back the moment the other group answers theirs. Momentum carries across the cut when it is a question that moves you, and dies when it is a timer.
Give the quiet group something to hold
Whoever is not on screen should still have a decision in their hands: what to watch, what to prepare, what to spend. It does not need to be resolved while they wait. It needs to be theirs, so that when you come back they already know what they are doing.
Let the two halves hear each other
One sound, one rumour, one light going out in a window across the street. Anything that lands in both scenes turns two separate games back into one game with two rooms in it. This is the whole trick, and it costs one sentence.
Rejoin early, not neatly
Do not wait for the elegant convergence. Once both groups know something the other needs, put them in the same place and let them tell each other. The retelling is usually the best part of the night and it only works if the party gets to do it instead of you.
