Painting a texture onto another using "Layered textures"


Hello! I’m basically trying to do this: but without Vray. I have Maya 2017 with the Arnold renderer.

To explain, I want to have a number of textures (in this case two) on my object, and with a third node, to modify the alpha of one texture. My problem is that I don’t know how to directly paint on my object and store it into my third node. This is how I’m trying to do it:

The two ramps simulate two textures and the “file-pcube” is just a baked texture I’ve made of what I painted with black/white on the object, since otherwise it does not affect the alpha channel at all. The lambert3 is just there so I can test the things I paint when I apply it to the object.

I’m not very experienced with the graph editor and shaders, so I’ll just say the issues I’m having.
Firstly, I still have no idea how this 3D Paint Tool works. Sometimes it does, sometimes it doesn’t (red x pointer) and I don’t understand what conditions it needs.
Secondly, I can only see the two textures with the alpha map in the Maya Software and Hardware renderer. I cannot see it in my viewports or the Arnold renderer (I’ve actually got it to run in Arnold once but I can’t seem to find the combinations of steps that made it to work).

What am I doing wrong? What should I be doing instead? Sorry for the lack of experience.