First off, how do you put yourself in the same frame twice like your talking to your twin in PP2?
the simplist way to do this is to film person 1 - making sure they are only in the left part of the shot.. then film person 2, making sure there only in the right side of the shot (an easy way of doing this is using something as a refence -like a lampost) - always making sure there is no movment in the camera... then in the edit- overlay the to clips and crop one of them 50%- and thats it.. it gets harder when you want to have movment in the shot, from the camera or from the people...
hope that makes sense -cos im quite drunk
But it's harder to get right in Premiere (unless they've added some new stuff in V2 that I haven't found yet) than it is in something like After Effects.
In Premiere your join between the two clips is typically a hard line which will show up any change in lighting. In AE you can 'feather' or graduate the join making it look much better.