Hey, sorry that it took that much time 
I tried both with different results.
The first option with 0,01 multiplier and using the red channel works with the mask.
I can see an effect. After all it is not the effect I hoped for but it is a start.
The material should replace the other material but it just blends over in a bad way.
Anyway I guess this is cause I messed up the blending too?
I had some problems with the second option as I do not find " subtract object location RG add
Texture size / 2". Can you please explain or send an image what do you mean with object location RG and Texture Size?
This would be really helpful.
I added some images to explain what I mean.
There is one with that what I made out of each of your solutions.
One shows the example which isn’t really helpful I guess.
The last one shows how I blend those both materials, anyway the goal
is that the mask is saying where one material is shown, not blended.
Btw. next time I will test it faster