Or the software can just ignore people that vote both up or both down. The more interesting and 'telling' votes are those who agree but vote down, or disagree but vote up. Of course, it would behoove pg not to tell the secret, otherwise people would simply game the voting.

Overengineer much? The current system works.

The best way to disagree is already possible: reply to a comment by stating your opinion. Off course that can have negative effects on signal/noise ratio (or not?).