I think it was HifiChoice in the UK a couple of years ago did some true scientific measuring of biwire vs. non-biwire (assuming decent jumpers) and found no measurable difference whatsoever anywhere and concluded that it made no difference sonically. I have tried biwire vs non-biwire, and single wire tops, bottoms and diagonally in two of my systems (one of which has been described as a "must biwire for best results" type deal) and cannot tell the difference.
Others swear it makes a difference. Who knows.
Others swear it makes a difference. Who knows.