I guess this would depend on the skimmer, you tank and water flow. If you have good water flow, a drop well and a skimmer of suitable capacity, then you would probably be wise to save your money and skip the second skimmer.
On the other hand, if you have a situation where your current skimmer doesn't get all the water, or is in a low flow aquarium, or under capacity for the tank, then it would be of benefit.
I'd be more concerned with the wasted power consumption and additional noise generated. I have one skimmer on my 55g tank and it picks up perhaps 5mm of sludge per week, reducing as time goes on.
You really need to give us more information.