I saw a couple kids riding really bizarre tandem bicycle the other day. Normally, the seats are inline, but on this one the seats were mounted on opposite sides of the bike! There were two sets of handle bars, though it appeared that only one of them was linked to the front wheel. There were also two sets of pedals, in a crankshaft sort of arrangement.
I wish I could have gotten a photo of it. It must have been custom made, because I haven’t found anything like it online.
Seems like it would be necessary to get on and off of it simultaneously, to prevent tipping over. And riding alone would be impossible – you’d need the other rider to maintain balance.