If the two tracks were played separately there should be little problem, however, two instruments playing exactly the same parts can have phasing issues.
As we all know, phasing problems occur when the same frequencies are 180 degrees out of phase. So the same instrument playing the same part could indeed have some frequencies out phase, but I think at the worst, it would just give a chorusing effect. Don't know for sure.
If it's single note lines, it could maybe be worse. Heh heh, never ran into it before, it would be interesting to hear.
