I'm running a 4th ed game, but I don't think that makes a difference in this case. Last session my party decided to split up for a couple tasks. The hybrid bard/rogue and the avenger (affectionately known as Team Ninja) decided that since they were the only party members with stealth they should follow the brainwashed paladins and only call on the PCs if they were in trouble. After that they were the only ones present for an interrogation because they were the only characters with enough Religion skill to make the Discern Lies ritual worthwhile. All the while, the rest of the party sat around and waited.
From a strategic perspective, what they're doing makes sense. I know these players aren't trying to isolate themselves to take over the spotlight. The rest of the party agrees with what they're doing. But the rest of the party is also getting bored.
What I'd like help with is coming up with ways to entertain the rest of the party. I'm not looking for contrivances to keep the party together all the time. I actually like watching new party dynamics emerge as the players have to adjust to their strikers being away.
Here's the part I'm finding difficult. I can easily add tasks to their queue, that's not a problem. Adding tasks that have to take place at the same time is what's stumping me. What prevents Team Ninja from doing their scene and then rejoining the party to handle what's left?
The other troublesome part is that I did not plan for the PCs to act this way. They just started doing it on their own. Had they told me last week they were going to split up, I could have come up with something else to happen. Hell, had they told me that the second part would involve the same split I would have thrown trouble at them earlier, giving more play time to the reinforcements.
So what are some things I can do to keep everyone entertained when the party unexpectedly splits?
-- update --
Just to clarify, I agree that I need to pay more attention to the players who are off screen. Ideally they'll be able to tell me what they're up to. But, if they say their characters wait around for 6 hours, what are some examples of things I can surprise them with?
In a previous game I managed this really well. I wrote out a whole bunch of letters and communications. Some had specific destinations. Others had to be stumbled upon. When a player was bored, I handed him the next letter or came up with an excuse for him to intercept someone else's message. The difference is in that game, the players managed a castle. They were always in one place. In this game my players are on the move. I don't know where they'll be. And they spend most of their time in disguise. Messages from world events aren't likely to reach these PCs. I could start a new plot each time the players split, but (separate issue) they've told me they have too many open plot threads and would like to resolve some before opening anything new. Given that, I'm not sure what I should be throwing at players who are content to wait in the bar.