There are many Oil catch cans out there on the market to choose from and a substantial price difference. The statement you get what you pay for does hit the mark when it comes to PCV Oil Catch Cans. Many Oil Catch Cans who are on the lower end of the price bracket are single port cans. This means they have 1 "in" valve or port and 1 "out" valve. With all PCV Oil Catch Cans, the "in" port is always connected to the Positive Crankcase Valve. This is where the dirty air exits the engine and goes into the intake manifold to recirculate back into the engine. Back in the day before air pollution control, these gases were let out into the atmosphere. Routinely, there was a tube that pointed down that allowed the oily residue drip out. That is why there used to be a lot of oil on the roads.
With a Single Valve PCV Oil Catch can the "out" valve generally goes to the intake manifold. You basically place an oil catch can to "catch" or clean the dirty air before it gets recirculated back into the engine. While in idle the intake manifold vacuum is used as the suction source to evacuate the harmful compounds entering as blow-by from the crankcase before they enter back into the engine that will cause coaking issues. As you accelerate that vacuum drops to zero and no evacuation takes place. It is during these periods that the water, gas, acid, and carbon settle and mix with the oil. So with a Single Valve PCV Oil Catch Can the only time you are "catching" with your can is when the vehicle is in idle.
With a Dual Catch Can the "in" port and first "out" port are the same. The second "out" port works to use the vacuum produced after the throttle is used. A Dual Catch Can is used to tap into an alternate evacuation suction source. This source is usually at a location just in front, or upstream of the throttle body. The second dual valve connects to your cold assembly intake or if it a turbo application connect it to your turbo inlet. Most modern vehicles have inlets but you may be required to drill into your existing intake tube and insert a hose barb fitting. With the use of one-way check valves (The GEN 2 Oil Catch Can provides internal check valves) to open and close this will always default to the strongest suction source at the time. This provides full-time crankcase evacuation and provides continuous cleaning.
With dual "out" ports or valves in conjunction with check valves (to prevent any backflow through the PCV system and the Catch Can itself) the first "in" valve will use the intake manifold vacuum for evacuation suction when at idle. The second "out" valve will tap into the area in front of the throttle body for the suction needed to continue evacuation when accelerating or going WOT (wide open throttle).