Not all velomobiles have three (or four) wheels.  Some photos of 2-wheeled velomobiles can be seen here.  

Streamliners (video on YouTube) and the "Streetliner" have just two wheels.  

Many of the high-speed faired racing HPV's run on two wheels, such as the Bülk (film clip on YouTube), the 2002, 2003, 2004, 2005, and 2006 Battle Mountain contestants, and the world's 24-hour cycling record holder.  

Our tandem photo collection includes back-to-back tandem bikes, tandem recumbents, and a side-by-side 2-wheeled tandem bicycle.  

Tandem trikes can be side-by-side, back-to-back, or front-to-back.  Here's more information.  The best list of trikes, including tandem trikes, is Mike's.  

Many unusual multi-rider vehicles are listed here.  The ZEM is available in a 4-person version, and the Conference Bike seats seven riders.  The Fietscafe (Bike cafe) carries 16 riders, of whom 10 are pedaling.  

A community art project in the U.S. brought a a pedal bus to many cities in 2006 to 2007.

More HPV bus information: The Fietsliefhebber (bike-lover) page that provided the history of the Fietscafe also tells of several other bike buses. You can rent the TrapTap (pedal tap) 15-rider human-powered beer bus near Valkenswaard, Brabant, NL. They've also been available for purchase since 2002. Roermond NL has 't Trapcafe (pedal cafe). In Helmond, there's the PartyBike. And a link on the PartyFiets site brings us to a collective representing a group of party bus rental and sales opportunities. And someone offers a pedal-powered custom bus with bench seating, but with very little information about it.

Making a chain of trikes was carried to an extreme by Hase in Germany, when they set a record for tandem riding with a 150-meter snake of 93 linked trikes in the summer of 2007.  Ricksycles in Ontario, Canada now offers this chaining option on a side-by-side tandem trike, and Lightfoot Cycles sells a "Tandemizer" hitch to allow its trikes and velomobiles to be linked. Greenspeed's new new "Anura" delta trike brings this flexible tandem-building option to even more corners of the world, and Sinner in the Netherlands provides a tandem kit for its "Comfort" delta trike. Sun's EZ-3 trikes can be linked behind any of the others, and there is a hitch available that allows them to take the lead position. We now have an Anura and a Kettwiesel that we ride as a tandem, and we have added some photos and videos to this site. Advantages for the riders include independent pedaling, the ability to ride either solo or tandem (even during the same tour), and easier transport of two separate trikes. Bikes can also be linked side-by-side to make two-person, four-wheeled vehicles, such as the Quadribent.  

If you're in Minneapolis (mostly in the spring, summer, and fall, but sometimes in the winter), you can ride a tandem bike around the lakes, drive a Pedal Surrey past Minnehaha Falls, rent the Pedal Pub and party with a dozen or more friends along a scenic city route, or hop on a Conference Bike for a cruise through the city scene.

 Click on the "Pedal Pub" picture for photos of its introduction to Minneapolis, or go to the YouTube Fietscafe playlist.