I was wondering because I have a certain Pokemon I like (i bet you can guess what it is) and I have black and black 2. Can I get the Pokemon on both games, or will it mess them up?
It saves automatically once you make a save, if you even try turning your DS of at the exact moment of trading, you wont get away with it ;)
It automatically saves after a trade, so it is unable to clone like that. There are methods by turning off your system at a certain point in the trade.
Source: Experience and Bulbapedia in the 'List of activities' section.