So here's my results so far:
Three plantings of corn. First one a 4x4 block of silver queen, with 15 plants. Every plant produced one ear, but some of the ears did not get all the way filled out.
Second one some bicolor. 4x4 block with 14 plants. It was very hot and we left town for a week. It didn't get watered and I think got stunted. Started setting ears when it was very tiny. Almost all the plants made one ear, but the ears were small.
Third one a 3' diameter circle of Silver Queen with 16 plants. Despite all the heat, this planting has done the best. It is very tall and leafy. Every plant has one ear. We just ate the first two tonight. They were good sized ears of corn, filled out all the way to the top.
So clearly in rich soil, with plenty of water, you can grow corn crowded like this.
In people's experience, if the corn plants were spaced out more like the recommendations, could they have produced two ears per plant? Are there other corn varieties that more reliably produce two ears per plant?