Granted, we may be slightly biased on this, but in our opinion two is always better than one. Dual nozzle systems are more versatile and deliver higher performance because you have two individual programmable nozzles that come from opposing directions. Keep in mind the cost of a dual nozzle system is only 50% greater than a single, for double the performance.
But do you really need two? A single nozzle may be perfectly suitable if you are doing mainly three axis work using smaller cutters with fairly light cutter engagement. Whenever 4th or 5th axis work, complex fixturing, and/or heavier, deeper cutting is involved, we strongly recommend dual nozzles. The workpiece constantly rotating to different orientations can obstruct the coolant stream from one nozzle while providing a clear path for the other. The same can be true for complex fixturing or milling around the perimeter of a part. Dual nozzles address these situations perfectly, and are even more effective at chip evacuation from drills, endmills, pockets, etc.