All GMs know that adventuring is supposed to be done as a group.
After all, it's our job to keep players engaged and having fun in the game. That's hard to do when a player is just sitting there because his character isn't involved in what's going on. And what's worse is that they won't just sit there quietly, they'll instead heckle, or worse offer advice. Table talk is to be expected, and when they're all together as a group can be passed off as the characters talking to one another. But when the player's character isn't there, table talk is disruptive.
What to do?
Over the years as a GM, I've found some ways of managing split parties that work well for me. Give them a try, you might find they work well for you, too.
Don't Try To Prevent Splitting
No player likes to be railroaded into making a particular decision. In fact, the more that you try to steer them away from a decision, the more likely they are to make it just to be obstinate. So when they're talking about whether to split the party or stay together, just let them hash it out themselves. They might come up with good reasons to stay together.
Probably not, but it never hurts to hope. The only sure thing is that if you insist they stay together, they either won't or they will but the players themselves will be unhappy about it. And the name of the game is to have fun.
So if they do split, it follows that you have to switch the narrative from group to group at some point. One of the problems with splitting is some players having way too much time on their hands because the narrative doesn't get around to them quickly enough.
Part of the art of handling party splitting is knowing when to switch between groups.
Switch During Skill Checks
Computers have an interesting approach to multitasking. The basic idea is that if a program is going to be doing something that will take some time, even just a little time, the computer switches to another program while the first program is waiting for its task to finish.
With split parties, skill checks make a convenient place to switch. The sort of skill check to use is one where the character would expect to spend some time on the task. Searching for secret doors, building a barricade, deciphering runes, etc.
Ask them for the skill roll, note the result, and switch to another group, saying that you'll get back to them when their character finishes.
Switch After A Major Discovery
If one group makes a major discovery, switch to another group to give the first time to discuss the ramifications of the discovery. That's all in-character discussion, and you'll probably have little input into it, so handle what another group needs to do. Get back to the first group when they're ready to move beyond the discussion phase.
Switch At Dramatic Moments
If the players are going to split the party on me, I figure they can stand a little dramatic tension. So at particularly dramatic moments, I'll switch away from a group and make them anticipate the outcome.
For example, in a Call of Cthulhu game, one of the group split off from the rest (she'd already been half insane by that time) in order to try to find a hidden room in the basement. I described the nature of the hidden door she found, described her opening it, and then switched to another group.
Evil, I know, but GMs have to have fun, too.
Switch After A Few Minutes
If none of the above apply, and one group has had the narrative for around 5 minutes, switch anyway. Don't leave the other players hanging longer than you absolutely have to.
Work Everyone Into The Plot
There's nothing worse than being forgotten about, or not being in on the climax of the plot because of a party split. While you might think that a player who goes off on their own deserves missing the climax, it's your job as GM to make sure that everyone has fun. Work their going off on their own into the plot, such that everyone comes together at an appropriately dramatic moment.
If someone has gone exploring secret passages while everyone else is storming the mad scientist's laboratory, then when the mad scientist has everyone else on the ropes, have the lost character pop out of a nearby secret door.
Don't Be Afraid To Force Joins
Just because a party splits, doesn't mean it has to stay split. Especially if you have more than just two groups. When the plot seems to call for it, bring the groups together. They may decide to go their own way again, but most of the time they stay together if the joining doesn't seem totally artificial.
So the next time your players start talking about splitting the party, don't cringe or fight it, just smile and let them do what they want. That's why we're in the hot seat, to deal with whatever they might do and make sure everyone has a good time anyway.