You've got it. Advantage is something in my wheelhouse. Standard axle shafts have two options of manufacture: Forging or machining down from drill stock a piece that is OD of the flange or greater. Forging is the only option between those two that makes any financial sense.
Forging has its own hardships for manufacture. Material choice must be able to handle the forging operation without grain growth, too much decarb, etc... Then there is the Heat Treat operation. Heat treating a part when there is a large OD change generally gives some sort of differential heat treat properties near the juncture as well as major differences between the two extremes due to soak time and quench time. Same as tempering differences. Salt bath processing can help with this, but it is an inalienable fact and can only be diminished and not brought to zero. Think HAZ zone stuff to start figuring it out.
Note that differential Heat Treating can be a desired quality sometimes, and should not be thought of as only bad.
The double splined setup eliminates almost all of those issues. You also have free reign of material choice from drill stock that is just above OD of your largest spline. You can now get a completely uniform heat treat of the entire shaft. You can do it starting with centerless ground drill stock that can go directly into your swiss machine or turning center. Straightening the shaft after HT is going to be much more uniform than one with an area that will not be as predictable as another.
The downside is machine time, extra parts, and generally better material. These axles I just put in are 300m with 300m drive plates, and the small caps are aluminum. You've got 2 extra sets of splines to broach, 4 threaded holes, oring grooves, snap ring turning etc...
Nah. Really its a toy motor. Nothing big at all. My factory shafts also did awesome, but I saw they weren't super happy so lets try these bad boys.
