I am far from an expert, but I do understand business and physics... but not at keeping my mouth shut. Strange...
There are several reasons for a manufacturer to use two different steels. One is because it either looks, or is percieved by the buying public to be cool. Another is the "baffle them with BS" where marketing dresses up a pig and says this is what you want to take to the dance. Classic example is when Heinz ketchup was first testing their product everyone complained that it was too hard to get out of the bottle. Marketing took that negative and sold it as a positive, "ours is slow to make it so much better than everyone else's". It's F'ing ketchup! Anyway, similar behavior in the knife world would be " we only use the finest 8Cr13MoV steel available". To a BF member that is a "Cheap chinese steel declaration" but to the general public, it is often interpreted as "oooohh, that must be really good steel".
So that is the ugly side of the business. On the smarter side, the manufacturer is reducing his costs. A knife has to have so much steel to make the blade. You can't alter how much is needed. That is a constant quantity we will call X. But you can alter the costs of the steel you use to get that quantity X. For example - and I am pulling these numbers out of thin air to demonstrate. Don't jump on me if you pay different prices. I've never bought a pound of raw steel in my life.
If Blade X is made out of 100% M390 steel, it costs $16 per unit to make.
If blade X is made out of 100% 440A steel, it costs $2 to make.
So what happens if the manufacturer combines the 2 steels? Makes the edge out of M390 and the rest of the blade out of 440A? For ease of calculations lets say the quantities used are 25% M390 for the edge, 75% 400A for the rest of the blade.
Knife X = .25 x $16 (m390 steel) + .75 x $2 (440A steel)
Knife X = $4 + 1.50
Knife X = $5.50 when made with 25% M390 steel and 75% 440A steel.
So for roughly 1/3 of the materials cost of making the knife out of M390, the manufacturer can still claim that his blade is made with M390 steel. He could also claim the edge is 100% M390 if he wishes to. Plus being able to make 4 knives out of the same quantity of M390 steel if he made it 100% M390. Keep in mind, these are just the cost of materials. It may or may not make sense for a knife maker to do this. The money you save on materials could get eaten up very quickly in labor to make the steel blank in the first place. All part of the overall costs and pricing calculations that goes into running a business.
There is also the "physics" side of 2 steel composition blades. First, 2 steels combined is the definition of damascus steel and has been done for hundreds of years at least. Japan was also making multi-layers blades. Both regions produced blades with legendary cutting capabilities. Why?
Normally the knife maker will combine a soft steel with a harder steel, and fold them repeatedly to get a multi-layered steel to fashion the blade out of. (Wootz Damascus is completely different and I won't go into it) The reason for the hard and soft steels is that they wear differently, the soft steel eroding away faster than the hard steel. What this does is create "mirco-edges" in the steel. Mini-serrations if you will. This would provide 2 conditions that a single steel blade could not provide. One is additional cutting edges in the steel, and the other is less contact area. Then the soft steel wears away faster there are miniature depressions or ridges in the blade. Those depressions lead to less surface area which leads to less drag on the blade as it slices.
For ease of manufacture of 2 steel blades, one of the best ways ways to get a premium edge steel is to make a steel sandwich. Say you want 2/3 of the knife to be 440A, and 1/3 of the knife M390, which you want for the edge. The easiest way to create that is to do a 3 layer blank. If you take the 2 different steels all the same thickness, then all you have to do is bottom layer 440A, middle layer M390, top layer 440A. Brought up to temp, and hammered or rolled to size, what you get is a (hopefully) uniform steel with a M390 center. IF everything works as it should and welds properly, it would not matter which side of the blank you put the edge on. As long as it was ground to the center, you should have a M390 edge.
