Yes, I built it from two single bike frames and a few pieces of tube. I was 16 or 17 and wanted a tandem, and with no money it was the thing to do. It was a little flexable, but it could be ridden safely.
For some good infomation on building your own tandem take a look at Sheldon Brown's web page at: http://www.sheldonbrown.com/tandem/
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." - Arthur Conan Doyle