Here's a fun explanation, from a British cycling website.
Quote:
Tandems
In tandem means one behind the other and in cycling that confers several advantages, most of all with regard to wind resistance. On a tandem you have the power output of two and only a bit more drag than one, so tandems go better into a headwind, downhill and on flat roads. Since their weight is also less than double, tandems ought also to go a bit better uphill; but they don’t and this is a source of much debate. The most likely explanation is that you don’t actually get double the power, especially uphill, when each rider tries to move the bike in his own particular way and must instead reach a compromise with his partner. This theory is confirmed by the observation that well-matched riders can go uphill as fast on a tandem.