Each rider controls one wheel. Pedal together and you move in a straight line forward or backward. Anything else is a turn including a spin in place by pedaling in opposite directions. I guessed a top speed of around 7mph would be a good start. I set the gear ratio accordingly. It was designed for kids around 5' tall, but it can easily handle taller. I am 5'6" and about 160lbs. I rode it with my daughter and it worked well.
This instructable details how to make a basic version. I designed everything in Pro/E. I have exported all the parts to DXF format so you can access with free software like QCAD. The drawings have basic dimensions for overall reference. They are 1:1 so you can scale off them or add additional dimensions. There are also PDFs too. I used my homemade CNC router so I added a lot of details that are not required. Need more info, detail, pictures...just ask.
The project took me about 16-20 hours over 3 weeks to build. A lot of that was design and testing, so it could take you less time. I had a lot of left over and found material that I used. If you started from scratch this could cost $150 to $200 to build. The bearings and drive train parts are the most expensive. I bought them at a surplus store. Hopefully you can find them cheaper.
The design worked pretty much as intended without any major problems. I did learn a few things along the way. I added "possible improvements" sections to each step that might make your version better and easier to build.
The working name is SteamRoller due to the steampunk look. The kids like Wheels of Doom and Portable Porch Swing. Does anyone else have any suggestions?