Please note the following fact about dehumidifiers:
A dehumidifier is basically a room air conditioner which works by cooling the inlet air down just to the point where the water dissolved in it begins to condense out of solution. This temperature is called the dew point temperature. It does this by blowing the air over a set of cold tubes with refrigerant in them; the water vapor condenses on the tubes as liquid water which drips off the tubes into a catchment tank and the refrigerant carries away the heat of vaporization. This heat is then dumped back into the room by blowing more air over a second set of heat transfer tubes through with the warmed refrigerant is circulated.
A room air conditioner does exactly the same thing except that in this case, the heat from the warm refrigerant is exhausted outside the room and the temperature of the cold coils where the water is condensed is low enough to actually freeze the water solid if it does not promptly drip off those coils. Both these features are there to maximize the cooling effect of the device on the room; the dehumidification effect is an additional bonus.
This means that although you can improve the effectiveness of a swamp cooler by dehumidifying its output, you can't get the room air temperature below the dew point of the air in this way- whereas you can with a conventional room AC unit that exhausts its heat outside the room.
So, dehumidifying the output of a swamp cooler makes little sense compared to just replacing both of them with a conventional room AC unit.