There are quite a few tricks that use a similar premise - a deck of cards that has been cut in half. As well as the ones mentioned earlier, there is also Docc Hilford's 'Split Up', which is rather nice, and particularly good for pocket management purposed, as you only carry half a deck!
However, the effect Brian T has is almost certainly the one usually referred to as 'Split Deck'. It comes in two forms, that I know of. In one, as Marvell describes, a card is chosen from each half of the deck. The two halves match.
The other version is the one Tenko mentioned, where the two halves do not match, so it looks like the trick has gone wrong. Then it turns out that the magician's prediction is a half and half card that matches both selections.
The former is a much 'straighter' version of the effect. It doesn't require a prediction card and can be played in a number of ways, including as a mental effect. The latter is more of a comedy 'sucker' trick and, if it's the version Brian has got, he'll need to find the special prediction card too (or make one).
Here's a performance only video of the first version:
http://uk.youtube.com/watch?v=DFeQwYK92Ww