(Dang, I'm going to regret getting into this.)
Tiffany, there is no easy
way to accomplish this. When two bicycles are assembled side-by-side into a four-wheel "sociable", the matter of steering geometry
inevitably crops up.
Steering geometry means that both front wheels need to play well together. (Google "Ackerman".) This is not a slap-it-together-with-a-screwdriver sort'a thing.
I have ridden this sort of vehicle on the Playa, and it worked fabulously. But the guy who built those sociables had the technical background for it.
For a non-technical person to build such a vehicle in the next two weeks, with the bicycle you have now, and an other one similar to it, may not be realistic.
(I really need to NOT hit "post" now.)