I'm sure everyone here is familiar with the late Sheldon Brown and his wonderful online resource. The other day at work I was reading his homebuilt tandem page and got the itch.
<
Does anyone have any experience doing this? I have Brazing equipment, but didn't realize it was strong enough (assumed bikes need welds, but what do I know?). I'm planning on using two old road bikes (cheep/free schwinn or raleigh) for style, and a steel mountain fork and wheels for strength. I've got an old donor mountain bike already, and I'll use the cassette and rear derailleur for a simple 7 speed drivetrain. No front derailleurs or crazy crossover drive stuff.

I know it's a little crazy, but read the last line of Sheldon's article and you'll get it.
http://sheldonbrown.com/tandem-build.html