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.
I vote for two skimmers.
gg=alice