It would depend on what is around your pool area. I have 2 skimmers in a 15x27 pool because I have a line of pine trees next to it (not mine or they'd be burned down by now! :lol: ) and this keeps the pool fairly clean of the debris from these trees. I would think it would more efficient no matter what at keeping the pool clean...just my opinion!
We only had one skimmer in our 1946 20'x40' IG steel pool. It was retrofitted in sometime in the '60's and required a cutting torch to go through 5/8" marine battle plate so we only did one. Most of the time it worked just fine, but occasionally stuff would get hung up if there was a steady wind pattern from the wrong direction. Eventually the weather changed and it would get it. We lived with it just fine, since in fair weather it stayed perfectly clean. I now have two, and they do do a better job.
I have only one and would prefer to have two especially as our high wind directions change throughout the year. In order to keep the pool cleared I have to use an add on Pool Skim that uses one of the returns and in Fall, because of so many leaves, two Pool Skims that uses another return. For one or two Pool Skims the flow has to be reduced to the other returns for enough flow to Pool Skims.
If you have enough leaves going into the pool during Fall, even with two skimmers, a Pool Skim can come in handy, as it holds much more leaves than the skimmers do and if it gets full it doesn't affect the suction like a clogged skimmer will.