Paint existing material instance on top of other material instance

I have a question. I Found these two beautiful materials on megascan and I want to use one as the main material for my landscape and I want to paint the other material on top of the main material. Now they are both material instances, so my question is: How do I paint this material instance on top of my other material instance. Btw I just started a couple of weeks ago, still a bit of a noob.

Thanks!