I'm aware of sites like GovTrack and VoteSmart that let you examine any given Senator's or Representative's votes, but what I'd really like to do is get a side-by-side vote-by-vote comparison of the two Senators from my state. I want to zero in on the votes where they were on opposite sides. Of course, I'd like this for the two Representatives from my state, too, or for the general case of any two members of the Senate or House. 

I think an on-line tool for this would be incredibly useful, but I'd settle for a way to download their respective records so that I could pull them into a spreadsheet and line them up with each other.