I don't know about newer Harley's, but older ones used an ignition that fired both cylinders at once. The cylinder that's fired on the exhaust stroke is not supposed to have any fuel mixture in it. Just like the Japanese four cylinders. But the Harley's, being air-cooled and having pistons the size of coffee cans, may have some fuel in that exhaust stroke, or the intake valve may start to open with some cams.
Someone makes a kit that fires each cylinder separately. Greatly improves the idle. And may stop most vibration.