One of the early making projects I worked on with my good friend Powermitten. It was a side by side bicycle, no shared steering, no shared gearing. It was just two bikes welded together.
Our goal was to create a fun bike to ride at Burning Man that could pack up easily. We ended up welding flanges onto the bikes and used metal bars to connect the bikes. They worked well, even when one bike broke the other was able to drive both frames!