If you're still looking, there's a Cannondale road tandem for only $680 on CL in northern NJ. (
http://newjersey.craigslist.org/bik/4402544383.html) It's probably older (bar-end shifters rather than STI), but should be a great deal and a great starter if it's the right size.
It's not too far from me if you're interested but need help with logistics.