Speaking as someone who has swung (or thrown) a hammer for a living: I think that there is some misunderstanding about 'swinging a 9 pound hammer in each hand'. This would not be possible if you were swinging a hammer properly. I suspect that John Henry was good enough to be able to swing a hammer both right and left handed, most hammer men are, but not BOTH at the same time.
'Cos here's how you throw a hammer (right handed).
You take the free end of the shaft in your left hand and hold the other end of the shaft with your hand right up against the head.
You position your feet correctly. Note that the motion starts at your feet, not your hips. Your ankles actually twist first, then your knees, hips, shoulders, arms and finally your wrists. It's a full body workout.
You literally 'throw' the hammer head at the target, as it moves you allow your right hand to slide down the shaft so at the moment of impact your two hands are together at the end of the shaft.
As the hammer rebounds, you slide your right hand back along the shaft to catch the hammer head.
Swinging horizontally is not a problem. You just raise the head a little above the target before you throw it.
It takes a bit more practice to swing the hammer left handed but the principle is the same.
Also, (like singing) it takes ALL of your concentration, and breath, to swing a hammer properly. So when you see someone singing AND swinging a hammer, they can be doing neither properly.
The hammer shaft, of course, must be kept perfectly smooth. A lot of hammer men keep a piece or two of broken glass handy so that they can smooth their hammer shaft.