You can't fan a double action revolver since the hand rotating the cylinder involves movement of the trigger as well as the hammer. Single actions rely on the hammer to do everything, and so you can just hold down the trigger and fan away on the hammer as it rotates the cylinder and falls, not interfacing with the trigger or sear components.
Double actions are more complicated. While you can cock the hammer to put them into single action mode and pull the trigger, the trigger needs to reset fully before the hammer can work with the hand to turn the cylinder again. In other words, when you hold down on the trigger and fan the hammer, the hammer can't move the hand and turn the cylinder, so there's no more cartridges getting in front of the firing pin.
I guess in theory, one could fan the gun if they reset the trigger into double action mode each time, but it's so tedious, especially when you have a trigger that cocks the hammer for you to achieve rapid fire anyway. Add to the fact that the fanning would cause considerable awkward wear on the sears and internals, perhaps even the cylinder, and that rapid fire is not generally condoned, and fanning DAs becomes all but pointless.
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