It says it is a low pressure? I don't understand why I can't just buy the same units I already have.
Regulalators have a high pressure side and a low pressure side. The high side takes the high pressure from the tank. This is where the regulators attach to each other and the tank. These are the side holes.
Then high pressure gas goes through the low pressure side to get dialed down to lower pressures. These are the top and bottom holes. They go to the keg or to a secondary regulator. Low pressure is dialed down to the low side for serving or to lower the gas pressure so a secondary (low pressure) regulator can be used.
So high pressure = input and low pressure = output
You can take the high pressure gauge off the dual primary regulator set up in that above image and add another 100 more primary regulators in line if you want. These would all be considered primary regulators because they can all handle the high pressure from the co2 tank.
Or, you can do a secondary regulator. So you would have to have a high pressure regulator on the co2 thank (to take the high pressure) then it will dial down the pressure to say 30psi where you would put a series of low pressure regulators (they only take a couple hundred lb of pressure vs the high pressure at 1000+/- psi)
The secondary regulator would connect in series through nipples or tubes, this depends on what you have installed. That second photo shows nipples, so a hose would connect to the inlet and outlet. The primary would feed low pressure gas to the series of secondaries. Now the secondaries control the serving pressure of the kegs.
Primary controls gas to secondary, secondary controls gas to keg.
You can do one primary and 4 secondary for 4 kegs
You can do 2 primary and have one go to a keg and the other go to a series of other secondary.
You can do it however. But you really only NEED one high pressure regulator connected to the co2 tank. The rest doesn't really matter. Secondary regulators are just "easier" and allow you to mount them on the side of the kegerator vs having a bunch connected awkwardly to a tank.
If you have that dual primary set up shown above with the red knobs, buy another one, take off the high pressure gauge, add a left threaded 1/4" nipple to the hole, take off the step of the other regulator (that connects to the co2 tank) and add it in line with the others through that nipple.