First one is correct, you can even have a fixed / freewheel combo with one on each side. Gears have to be close in size to allow chain slack to be taken up by t

First one is correct, you can even have a fixed / freewheel combo with one on each side. Gears have to be close in size to allow chain slack to be taken up by the axle slots. Second one would only work on a few bicycles as generally the front hub is narrower tan the rear.