Oftentimes, a director or writer will have a scene in mind, say a dialog, or a big event, where there's two or more important points he wants to get across at the same time, but unfortunately, are happening in two different places, or at such an angle that you can't get both at once. One solution is to just alternate between showing the two, while another is simply to use a Split Screen to show both at the same time.
This way, you have your cake and eat it too: You can have your explosion and the reactions to it all in one shot, or you can see both sides of a telephone conversation. Alternatively, you can use it to show only loosely related events that happen to be going on at the same time, such as in 24 or the Metal Gear Solid 4 example below.
It's also a very common device in multiplayer console video games, particularly first or third person shooters, allowing each player to get their own view. Golden Eye 1997 on the Nintendo64 was one of the earlier and most successful implementations of this in a genre which, up to then, had relied mostly on linked systems for multiplayer.
See also: Split-Screen Reaction
In the 2007 movie The Tracey Fragments uses splitscreens to a nauseating degree in order to portray the fragmented memory and the mental instability of the title character. Every single scene in the hour-and-a-half movie is composed of splitscreens. And unlike conventional 24-style splitscreens, this movie's splitscreens seems manic and almost random, they come in all shapes and sizes, float in and out of the shot, cover and overlap each other, appear in the middle of each other, etc. In one particular shot, there are 14 screens on simultaneously, and for the majority of the film, there are always 2-4 on screen.
In Jackie Brown, split screen is used when Jackie (Pam Grier) steals a gun from Max's (Robert Forster) car in order to protect herself from Ordell (Samuel L. Jackson). The split screen shows that Max discovers that his gun is missing from the glove compartment at the same moment that Jackie uses it to threaten Ordell. Jim Smith's book Tarantino (Virgin Film Guides) describes this as "a non-gimmicky and entirely story appropriate use of split screen - which might be a first".
Ang Lee's Hulk went into split screen on several occasions in an attempt to mimic the style of the comic books the movie was based on.
Lola Rennt did these not only for Split-Screen Phone Call, but also to capture plot critical parallel actions. These also served as extended wipes, with Lola's split screen appearing from screen right, then later wiping all the way across to the left.
An entire movie, Wicked, Wicked (1973), was filmed in "Duo-Vision" (split screen).
The 1963 movie A New Kind of Love used vertical split-screen to underscore parallels between a fashion show (attended by the female lead) and a simultaneously happening burlesque show (attended by the male lead).
Then it used horizontal split-screen to show in one frame the male lead peacefully sleeping, and in another frame, the female lead tossing and turning with frustration. The implications of Paul Newman being on the top (of the shot) have got to be purely intentional.
Used throughout the film Conversations with Other Women. One part of the screen usually shows the female lead, and the other the male lead (and sometimes other characters); the split screen is also sometimes used to show different periods. Only the last shot doesn't make use of a split screen, showing the two main characters together...even though we've been showed that they were apart. It's all very well-done though, and not gimmicky.
Taken to a comical level of absurdity at the end of The Last Remake Of Beau Geste, when Beau and Digby Geste do their Pinky Swear grip, over a distance of hundreds of miles, while cuddled up with their respective lovers - Digby and Isabel on the lawn of their Big Fancy House, and Beau and Flavia on the beach at the French Riviera.
Wildly experimental Soviet film Man With A Movie Camera uses this over and over again. Mismatched shots of trolley cars in the streets, a shot of a man lifting weights that cuts out the middle part of his body, a shot of a city square where the screen splits in two and the two sides rotate in opposite directions.
Spooks and 24 both had this effect as a signature of their styles.
CSI: Miami has started using those for lab sequences.
Sanctuary uses this quite frequently to compress sequences instead of using a montage. The split-screen method is somewhat similar to that used in Ang Lee's Hulk (mentioned above), which gives these sequences a (possibly unintentional) comic book vibe.
Averted by lampooning the above in The Young Ones episode "Bambi". No, it's not a split-screen camera-trick, the contestants are actually arranged one o' top the other. As Vyvyan proved by stamping on the contestant's head, below him.
Used on Top Gear, when two presenters (in separate cars) are on the phone to each other (if a Hard Cut from one to the other is not used instead)
The Amazing Race started using split screens in season 14 to seg between scenes.
The Price Is Right has a split screen that is very memorable among fans. From when the hour-long format became permanent in November 1975 to sometime in 1996, when a contestant spun the Showcase Showdown wheel, a split screen would show as the wheel slowed down with a headshot of the contestant inside of an arrow graphic that pointed to the wheel on the left side, and the wheel on the right. The fact that they were able to set this kind of a shot up without any sort of computer technology (or with very little technology), at least, until the technology came in the 1980's, makes this camera shot a Crowning Moment Of Awesome for the production crew.
Doctor Who had been using chroma-keying for special effects since at least 1970, so the technology wasn't exactly that exotic!
During Game Show Marathon in June 2006, R. Brian DiPirro brought the arrow graphic back, but it wasn't a true split screen. It was an arrow-shaped picture-in-picture shot. When he became Price's permanent director in March 2009, he brought this shot back for bonus spins.
The split screen returned in November 2007, but it's just a generic one now. However, since Season 38 began, if there's a spinoff, they do a triple split, with the spinning contestant on the left, the wheel in the middle, and the contestant in the lead on the right.
Get Smart has screen split into three parts - see episode with an airplane in season 4.
The Modern Family episode "The Late Show" opens with a three-way split-screen
The theatrical equivalent of this is "split scene", where two scenes are shown at once, sometimes with the spotlight alternating between the two.
Also, almost goes without saying, but many multiplayer video games (particularly set-top games, which often lack networking capabilities) use a split screen.
Metal Gear Solid 4 used this very well in two different scenes. The first occurs when you play Snake and fight off a horde of Gekkos, while Raiden keeps Vamp busy in a sword duel. While the Gekkos demand your full attention and you're unable to take more than a few peeks at Raiden's half of the screen, it works extremely well in evoking the feeling that you're playing the game with another player. A second instance occurs later in the game when Snake is crawling through the infamous microwave corridor. This time the controls are extremely simple which allows you to take a good look at the action in the upper half of the screen, which shows Snake's allies getting completely ripped apart while they are trying to buy him a minute or two more to reach his goal.
A rare Video Game example (The only other one apparently being the Metal Gear Solid example) in Fahrenheit (A.k.a. Indigo Prophecy). Splitscreens were used usually whenever there was a scene where the main character had to do something quickly before a villain found them.
Along with this, Heavy Rain utilises these extensively (Both games being made the same company), especially for tense scenes.
Imitated in comics, like in thisLoserz strip. (Although here, we're rather talking about panels.)
Lampshaded, of course, in Yu-Gi-Oh!: The Abridged Movie. Kaiba leaps up on top of his dragon monster to give a speech, but Yami Yugi just shouts that he can't hear Kaiba at that distance. Once the split screen shows up, Kaiba asks "There, is this better?" to which Yami Yugi responds, "Oh, a split screen. Yeah, that's much better."
Cartoons often subvert this, especially the phone call variant, by having a character reach across the divide.
South Park did this in "Spookyfish", deliberately showing the split screen line between Cartman and Evil Cartman.
Yin Yang Yo tends to Split Screen for the twins' combat scenes so viewers can see what's happening to both of them at once.
Split screen is used in Danny Phantom episode "Identity Crisis" where Danny, having split himself—one fun, one super—goes about their given tasks at the same time.
The Kung Fu Panda Holiday Special has a sequence where Po and his father are working in the kitchen for the holidays where the screen is progressively diced into smaller squares like a vegetable until it is seemingly swept into the pot.