Placing a windmill behind another one would be stupid. So, when you have a farm, you still capture a maximum of 50% of the energy that flows through the farm - only, you cover a bigger area and thus you get <= 50% of a bigger mass of moving air.
If windmills are dirt cheap, energy and land scarce/expensive, or transport losses large, a windmill in the wind shade of another windmill could still make economic sense, just as it can make economic sense to install solar cells on roofs in places far from the equator that often have cloud cover.