I don’t think the Mix shader is working properly if you connect two Volume scatter shaders into it, and output to Volume. With Fac-value of 0 or 1 it uses only the other Volume scatter node, which is correct. With any other value, the output is clearly 50/50 split between the shaders, no matter if the value is 0.999 or 0.001 or 0.5. I am using Cycles and Blender 3.6.1.
Background is that I am trying to build so-called double Heney-Greenstein phase function for scatterers in a volume. The Volume scatter node implements (at least almost) one Heney-Greenstein phase function where the Anisotropy parameter is the Heney-Greenstein parameter controlling scattering either to forward or backward. Combining two with unequal weights would allow me to have sort-of-decent approximation of Mie scattering. Approximate Rayleigh scattering already works quite nicely by combining two Volume scatter nodes with Anisotropy parameters 0.311 and -0.311 and equal weights, but the general double Heney-Greenstein would need unequal weights, and clearly the Mix shader is not working correctly with that.