Dual reality is a technique used by magicians everywhere, and yet, if you were to ask a magician to name a technique, I can't imagine it being the one they pick. Yet, if I had to guess, I'd say it is in fact the most common technique of all. It is used by close-up and card workers, stage illusionists, and most especially mentalists. If you see a TV magician do something that looks impossible, there is a good chance they are using Dual Reality. Possibly half of Derren Brown's TV "routines" involve it although no one but him and his victims could say for sure what is what and which is which.
"Well, what the hell is it?," you ask. When a magician says "dual reality" they do not mean the ever-present disparity between what the magician knows is happening and what the audience is supposed to believe is happening. No, they are referring specifically to the situation where there is an inequality of information within the audience itself, in such a way that different audience members witnessed different occurrences.
This disparity can be used in a variety of ways. The most obvious way is wherein the audience is "let in on a joke". Usually this means an audience member is singled out and information is hidden from them. For instance, the "secret" of a trick might be visible to all but that person. The most common version of this is the toilet paper trick. I have seen this performed live by at least two different men, and it pretty much always looks exactly like this. The last occasion really illustrates the principle. Firstly, (and this is common to all performances) the magician always told a joke just before tossing the wad over the spectator's shoulder. Thus, when the audience laughs at this audacious action, it appears to the victim that they are only laughing at the joke. But, even more audaciously, the magician endeavored to vanish an oversize solid metal nut (weighing at least ten pounds) in the same manner. Just before doing so, he said to the audience, "You guys better make a lot of noise if I manage to do it!" The spectator heard "You should give me some applause!" The audience heard "I need you to cover up the sound of the nut hitting the stage in a second." That is what dual reality is about.
And that's the only situation in which a magician might turn an audience into a group of willing stooges. A more interesting example of dual reality is one in which the audience's mistaken belief that it knows something the victim doesn't leads it to fool itself. Consider this effect by Richard Bellars. At one point, he openly says "All my stooges are in the audience." The idea here is to give the audience the impression that their interpretation of what they are seeing is also the magician's interpretation. It is their firm belief that Jonathan Ross is not going to see what he expects to see that leads them down the primrose path to buying his "mental force" NLP bullshit explanation.
Sometimes the disparity in information IS the entire trick. I know of one good effect that consists of nothing more than convincing a victim that received less information than everyone else that they had all the information. That is Jay Sankey's Memory Lapse from his DVD 22 Blows to the Head. I cannot show you a performance of this effect, because I know of nowhere on the internet a video is available, but it essentially consists of showing a card with shapes on it to a spectator and asking them to remember them all and then sign the card. The card is then given to some other spectators to look at. The victim is then asked to name the shapes, and lists the three shapes he saw. The other spectators show him the card he signed which has four shapes on it. Thus, it seems that the magician has caused the spectator to forget a shape. The real disparity of information here is that only the magician knows that the victim and the audience saw two different sets of shapes.
By far, though, the most popular use of dual reality is for television magic. Everyone has seen that trick that "everyone can participate in at home just by placing a finger on their television screen." You know, the one where you move to adjacent symbols one at a time as different choices are eliminated, but no matter where you started or what you choose, you'll end up on the same choice as everyone else. This sort of thing isn't as popular as it once was, since anyone can duplicate the trick just by repeating the magician's actions exactly, and confirm for themselves that their choice didn't matter. However, it is an excellent example of dual reality: It actually uses around 20 different realities distributed among thousands or hundreds of thousands of people.
More common these days is the "reality" TV effect, best perpetrated by Derren Brown, wherein lies are piled upon lies to thoroughly convince the TV audience that the people they see on TV are behaving a certain way for a certain reason, when in fact other explanations are perfectly reasonable. In fact, he sneakily admits a bit of expertise in the use of dual reality in the first episode of Trick of the Mind by hiding from a group of chess players the fact that they are playing against each other. As an exercise to the reader, in The Heist, isn't there another more reasonable explanation for why three out of four participants "spontaneously" hijack a bank truck than some cockamamie story about brainwashing? If you can come up with a simpler explanation, it is probably the right one. Brown is, before all else, a magician and a liar.
But then, those two things are really one and the same, are they not?