The term "efficient" is being used in two different ways. On Mr.X's side efficiency is being used to describe the skimmers ability to pull out DOC's where as on Jeff's side he's using it to describe power consumption vs the amount of DOCs pulled out. Mr.X is correct in stating that one skimmer does NOT effect the others ability to pull out DOCs it just causes there to be less DOCs which in turn causes the other skimmer to be unable to pull out as much because there simply is nothing left for it to skim. It doesn't make the skimmer less efficient, it does however mean you are powering a piece of equipment that isn't working at its full potential thusly making it "inefficient" to run. In summary, I side with Mr.X, two skimmers in a sump setup don't effect each other, they both effect the water so when one pulls all the DOCs it appears as if the other is less efficient only because there are no DOCs left to pull out and the skimmers did their job "efficiently" but it is "inefficient" power consumption wise to run a skimmer that isn't skimming or only skimming half the amount it could... Clear as Mud I'm sure.