If anybody has ever experimented with old-timey flanging or phasing using two tape decks, or using two tracks fed from a third mono source (speed variable using your thumb, like a turntable or tape deck reel), you might have discovered the difference between a stereo phase and when you sum it to mono. The panned tracks you spoke of earlier get their directionality in the stereo field by varying degrees of out-of-phaseness with each other. Only if both sides were positioned symmetrically from each other would they completely cancel out, which can be "tuned in" with panners. That's how those vocal eliminators work. In the old-timey setup I described earlier, when you sum a stereo phased/flanged signal to mono, you can hear the cancellation as a phaser-type sound, which just sounds like a variable filtering of the sound. In stereo, that same signal appears to move in the stereo field directionally, and creates a delightful fluctuation, more like a flanger (the name is derived from the tape flange or reel). Guitar players sometimes use two mono phaser or flanger pedals to create a "fake stereo" effect. It seems like the L/R tracks you speak of are already out (in) of phase with each other, since they're panned hard left and right, which centers them in the stereo field. You said they were the same signal panned hard L & R, which would simply center them, but when you put one out of phase, depending on the degree of out of phaseness, it would just cancel them out or change the directionality in the stereo field. Unless they are actually two different takes of the vocal, it doesn't seem like a viable idea. It seems that a similar effect could be achieved using a harmonizer, or by simply doubling the vocal in two takes with a really accurate singer (like Elton John did on some of his stuff).