Look up “American Speedster” Sidekick. https://americanspeedster.com/
You buy the plans and some hardware if you need it. The rest is PVC. I built one in 2011 for BM and it’s still usable after 12 trips to the playa.
the sidekick looks fun — how do you transport it? (a big part of the design we came up with is that the side-by-side can be (dis-)assembled into "just bikes" and fits on a regular bike rack.)