What is the best adhesive for sticking two pieces of cardboard together for Prototyping?

Alan Chenkin, Uses glue all the time, for fun and woodworking projects
At the Chenkin Workshop prototyping is the most fun.  when you build something you never built before,  there is usually some vocal and provocative discussion, and test-attempts at assembly – and even production concerns.  Much more exciting than building something off a plan.
Aleene's Repositionable Tacky Spray, 10-OunceELMERS Repositionable Mounting Spray Adhesive, 10 Oz, Clear (E454)Stencil Ease Repositionable Stencil Adhesive Spray - 4.4 oz. can
If you are prototyping, my guess is that you need the ability to re-position the piece. For that you might prefer a spray-on re-positionable adhesive. Here is a list of them from Amazon.com: repositionable adhesive spray
The original glue for this purpose was rubber cement.
- Rubber Cement, Repositionable, 4 oz
Make sure the work is clean, and free of dust. sometimes you can spray on one side to be adhered,  but you usually need to spray both sides, let them dry to a tacky state, and press together. Try a test piece, before you risk the assembly;  Too much cement may make a permanent bond, and too little may not make any bond.  Be sure to let it get tacky, almost dry, as this is a “glue to glue” adhesion.
Good luck with your prototyping!