I saw this and had no camera DUH! What I thought was duallies on a garden tractor turned out to be old tires filled with concrete with big carriage head bolts going through the center. I couldn't tell how it was done. The tires were touching or nearly touching each other. The outside center was all concrete with two carriage heads visible.
It was on an old wheel horse. If I see it again with someone around I'll ask if I can take pictures.