You have one pump. Under the driver seat, inside the frame rail.
A water sep. filter does that, separates the water from the diesel and also filters down to a certain micron rating, usually not very fine.
A post pump filter has a very fine micron rating and filters only since the water is already out.
A water sep filter can be pre pump or post, don't see why it matters. But a high eff. filter should be post pump because it's hard on the pump to PULL diesel through such a tight filter, but much easier to push through one.
Two filters like this are better than the stock filter. The stock filter is like having a donut and sushi shop in one. It might function, but it's not pretty.
Two dedicated units for their own job.
Not sure of micron ratings, but that's what most people run, or whatever brand cross they like.
Usually this accompanies a fuel system eliminating the stock fuel pick up assembly, larger suction lines, dual stock pumps or a bigger aftermarket pump, larger pressure lines to the heads which flow through then to a regulator and back to the tank and deleting the fuel bowl. Never seen it done without that.