Eariler this summer I built a tandem with two frames I picked up at garage sales. It still needs work, but very rideable as it is. Really fun to build, even more fun to ride.

http://webdisk.berkeley.edu/~tommyatkins/bicycle/tandem/images/tfullside.jpg

Thanks for the inspiration everyone.

More pictures and description here (http://webdisk.berkeley.edu/~tommyatkins/bicycle/tandem/tandem.html).

Looks like an economical homebrew tandem!
To stiffen up the frame a bit, would suggest running a large round or oval tube to connect the 2 bottom brackets,then you could attach an idler arm to take up the chain slack (like on some of the old same side drive department store/Schwinn tandems) and that should alleviate the problem with the connecting chain.
Nice project!

For that matter, a "wheel"-type idler, like those found on many recumbents, would probably work, and would be quieter than the "pulley"-type idler you see on cheapo tandems. Shoot, for a while there, a guy on eBay was selling bags of TEN idler wheels (just the plastic part) for, like $5 plus shipping. A skateboard wheel bearing fits perfectly inside one. Shoot, I could probably send you one for a buck or so shipping. Then you'd just need an adjustable bracket from which to hang it...