Let’s look at how it can be done fairly neatly.
Say you will be playing for two hours and there are four players.
We could slice that into 30 minutes per player!
So if one player splits off from the rest, they get around 30 minutes of solo play.
“But everyone has to wait for thirty minutes as one PC does their thing!?”
No one said that!
You can take that thirty minutes and breaking it into 5 minute chunks. Mark down the time when the solo PC starts their time. The solo PC gets to do stuff for roughly 5 minutes.
When done, note a five minute chunk has been used up, until they’ve used up their full 30. Do about 15 to 18 minutes with the main party, then the solo player again.
Try to end on a ‘not sure what will happen next!’ cliff hanger with the solo player at the end of each five minute chunk, to keep them hanging on for more. Try not to finish off with ‘Okay, you bought a sword…now back to the main party!’. Sometimes you will end up having to do this, but if you can stop at four minutes on a cliffhanger, don’t resolve it to do the full five minutes.
Sometimes a player wont even want to use their full time and will rejoin the party after completing their business. Though sometimes the PC is just not part of the party anymore – they are their own faction in the game world, as much as the party is. Though with more members in the party, the party is more likely to survive or spend less time sneaking, figuring ways to sneak or retreating!
“But I don’t want to watch that PC even for five minutes!”
Something has gone wrong here – you should be interested in your fellow players characters. Why did you invite the person if you are not interested in their creations?
“I don’t really like that other gamer!”
But you’re gaming with them anyway!? For goodness sake…it’s not splitting the party that is dysfunctional, it’s the social context of your games that is dysfunctional. More exactly the lack of social context!
I’m…I’m just going to go back to the functional groups and speak with them…
“But a split party gets in the way of my plot!”
Okay, I’m going to just link to a post I found recently here