How hard is stamping concrete?
Actually, what you want to do is a three step process.  You first need to pour the concrete into the forms and screed it. The second step is to float it and bring some of the butter out. The third step is to stamp the partially dried concrete.  The stamping can go pretty quick if you had two molds set up on a tamp or step board that can be tapped with a mallet. The working time can vary, temp, humidity etc, but you usually have an hour or two to do what you need to do. Ask the concrete company to give you a longer cure mix, just a bit wetter