Well, you're obviously not familiar with the recumbent world. Since you don't need to worry about pedal strike, independent pedaling and/or independent shifting is practical. For a back-to-back tandem, independent drivetrains eliminate the long (lllloooonnngggg) chain run from front to back. Here's an M5 carbon-fiber back-to-back:



If that's not wacky enough, my friend Gary Dinsmore built a both-wheel-drive, both-wheel-suspended, moving-(front)-bottom-bracket tandem. This is Gary riding alongside the original WYMS designer Bill Patterson with their spouses: