I've seen such a thing - I think it was on display at the cycle-hire shop next to the Seven Sisters visitor centre in Sussex. Don't recall if it was being offered for hire or just on show as an attention-getter. Looks mighty uncomfortable to ride, especially if the two riders are not evenly matched (or of ample girth!). And of course there's no way you can continue solo if your partner drops out. I'd say, stick with a tandem....