This is true and most PW makers avoid this recipe because it has been pretty well debunked as producing hard and soft layers together. The principle that defeats this myth is that at welding temperatures all the carbon from both materials will average out. You wind up with a medium carbon steel that may or may not work well, but it will have a pattern.
Using similar tool steels is a very good thing to do. To get the steels to look different, commonly, the smith will use an alloy that contains 2% or so of nickel which resists the etchant and produces the whiter layers. But, there are many combinations with other alloying elements that will produce contrasts. There are ways to produce a white layer in the same steel, simply folding it back onto itself, that will produce layers and contrasts, but that requires some really effective technique or gross inattention.
Given the correct edge geometry, mild steel will cut stuff. It won't cut very long compared to a hardened higher carbon steel, but it will cut things.
I know several very good folks who could accomplish welding two steels of significantly different characteristics. I was told it wasn't a good ideal to weld L6 to 52100. That was a very miserable but successful project that I will not likely do again. The knives are really quite nice though.
Maybe it's best to say that nothing is impossible. These same folks could (have done) blades where the layers are of different hardnesses. But, that was a deliberate experiment that required precise temperature controls that are not very common in the smithing world.