Here is what I remember: two molecules are bonded, or connected in some way (maybe not bonded in the usual sense of the word?) When they are separated, even when there are vast distances between them, they will both react if one molecule is stimulated - if something happens to one, the other molecule reacts. Like calls to like, except on a molecular level.
I cannot remember more details, or where I heard it. If anyone knows anything about this, please explain, or link me to an explanation. I'm trying to write a sci-fi story and this theory, if I haven't made it all up, would be an incredible help to me in basing my world on reality.