A neighbor of mine has two modern ones: "Buddy Bikes."
It's fun but very strange to ride. He can ride it solo, but I don't think I could. Leaning way over to go straight is too bizarre!
The comments at the websites mentioned in the previous post pretty well sum it up. My neighbor was trying to adapt its parts to a better frame but the franken-crank and BB was the stopper.
David Bean
Arlington, MA