What am I going to do with the water that comes out from the skimmer?
Single pump - Water goes to pump from skimmer and another pick up with only one discharge, from pump 1, of water to filtration, waterfall, tangible point return, etc. Unless discharge is split, then flow rate is also split.
Two pumps - More flexible, more choices on where to send water because you have two pump discharges, pump 1 and pump 2. Higher flow rates.