OK ive heard this effect many times, and now ive decided to know what its called and what i can use to achieve it.
The effect im talking about sounds like theres 2 guitars playing together in perfect harmony but each in different octaves or scales or something.... THe best and the only example i can remember it being used in is by Dimebag in Blue right turning Red @ about the 39/40 second mark....
if anyone could tell me plz! :)

cheers