Did a small search and came up with nothing even close to a production tandam velomobile. Im betting what you saw was a custom build. They are hardly unusual in some parts of the world. In Australian HPV races of the past, there was actually a cat. for them!
In the 2004 Australian International Pedal Prix, the following vehicle raced;
As you can see the rearmost rider is facing the tail, rather then the front like the foward rider. Picking up on this, one of my future team-mates went out and scared the $hit out of the rearmost riders during the early morning, riding this;
Coming out of a particuly fast section of the circuit, he stumbled on the tandam above, approaching the poor thing at twice the speed it was traveling, horn blazing away. He got closer and closer- close enough to make out the rearmost rider's face and features through the darkness. At this point the young man was, and this was obvious to my team-mate, screaming at his foward facing companion to "get out of the @#$%^&* way!" or something similar . So anyway my teammate changes direction at the last second, with the last thing he saw before finding clear air being a helpless (in that he had no steering control over the vehicle, unlike his foward facing companion) male, eyes bulging, mouth agape and an outline of his heart very much in his mouth .