As far as I know you can’t blend materials together that are in different slots as you have them shown on your image (that’s as far as I know - there are far far more expert people on here who I’m sure know how!). You can do it by mixing textures or shaders within a single material. This is probably simpler that what you’re trying to do.
In my example, I have two shaders which are mixed together on the Z axis and using a colour ramp to control the blend between the two. You can also blend on the Y or X axis in the same way, just change the connections. You could do the same with a single shader, but mix the inputs before hooking them into the shader.
I’m sure there are other ways to do this, but this is the way I know!! Hope it helps.
I’d also recommend this from creativeshrimp: https://www.creativeshrimp.com/texturing-mix-tutorial.html . It is for pre-Blender 2.8 but I think most of the principles should be the same. This provides a lot of good advice on mixing materials (and where I learnt the approach identified above).