A roleplaying mantra: Don't split the party.
It's been ingrained into our noggins for years; bad things happen when you break a team apart from a group of 4 or 6 people into two, or God forbid, three smaller squadrons. The popular media's full of examples why not to.
Hellboy and his boss get blindsided by old Mr. Clockwork Nazi in the first movie of his. Ihmotep grabs a pair of eyeballs from "Graverobbers R Us" in The Mummy. If I WATCHED horror movies (which I can't), I'd be able to quote more examples than I knew what to do with. It's just something you never do, period.
Of course, when you're sitting down to play a game of D&D, a lot of the same reasons apply, but you tack on a few more and remove the soundtrack mechanics. (Unless you have a DM who choreographs every moment of his game with music and swaps out sound effects tapes like a radio disc jockey, and good luck finding someone that dedicated or scatterbrained!) When you're playing D&D, splitting the team apart means that your smaller crew will go up against odds that they might not survive against. The party's wizard might get toasted by that Flame Jet trap without the benefit of your resident rogue skillmonkey, who (Wisely) partnered with the cleric.
And of course, let's not forget that splitting the party also makes it harder on the DM to boot.
Of course, if there's one thing that you should learn about me through these blog posts, it's that I'm not one to follow all the traditions. I cut my teeth on plenty of games where the DMs set us up against impossible odds and expected us to balk and run for cover. That would be the smart play, after all.
But remember who you're talking to here; The Defender of the Law of Awesome. And when faced with impossible odds, that's when I start thinking. And the oddball plans, ohhhh lord.
Maybe you can split the party and get away with it. If you're willing to put aside probability and take the long shot...and if the DM can accomodate.
There was an article similar to this a while back in Dragon magazine; talking about how you could set up solo encounters, to give your players a chance to shine and feel super-accomplished. You could easily apply the same mechanic to a scene where your party of adventurers are exploring a dungeon. So let's go ahead and set it up:
You've braved the top floor of the deranged Alchemist's Lair and survived the early traps and guardians. As you descend to the first basement level, though, you find yourself staring at two paths...
Ordinarily, your intrepid crew might go down one path and investigate the other later. I'm DMing a game where this is the case. But with a little re-arranging, you could make it so that there's a connected set of switches in front of a cumbersome iron portcullis in both corridors that glows with an eerie light: Teleportation powers fizzle in its presence, which means that the only way to get by is to press both switches simultaneously...a goal that requires splitting the party. They did something similar in Skies of Arcadia, an old RPG which is a hoot because it's pirates...on AIRSHIPS!
Or, alternatively, you could have a trap trigger the party's dispersal; say, a trap door drops two members of the squad to an alternate path that reconnects to the main one after some trouble and mayhem. I did that once, tossed my pals into a winding maze where one false step would cause them to plummet into a basement full up of hungry, inquisitive Gricks. To this day, I only have to say, "Chirrup?" and they all panic.
Of course, once they're split apart, whatever the method, you then have to decide what to do with them. Is your wizard on his lonesome? Swarm him with minions, let him go crazy with those area attack powers! Nothing gives a wizard a warm fuzzy quite as much as roasting an entire squadron of chittering foes in one blast. Does your Rogue have a buddy, such as a fighter, or his old friend the cleric? Task them with defeating a healthier guardian with lots of hit points, and see if they can't manage to flank and take down the big guy with some well-coordinated teamwork. Got a Barbarian big on resisting damage or just not caring? Toss him through a hallway of traps and make it easier for them to be destroyed. After he smashes the hammer dropping down to crack his skull open with his big swinging axe, you can bet he'll be feeling like the top dog.
Don't be afraid to split the party. With some minor planning on the DM's part, the players will ideally rise to the occasion, reach down inside of themselves, and surprise the Hell out of you with a trick you never saw coming. As usual, it's the moments of Awesome you don't expect that make the game worthwhile.
And that's the Iowa perspective.