Ok let start out simple first
Let say I have Layer 1 use a default piano patch from Logic Pro X and turn off all effects , reverb etc…so is very dry
So I can later
Layer 2 of just a recorded single scratching sound I do by scraping the nail of my finger on my wood floor
(note of I would have recorded maybe 4 layers of variation of the scratching sounds but trying to get the basic concepts with just two layers for a while until I grasp this)
And I want to combine both layers in a way where when I play very softly I can hear the piano fade into the other
So I But as I play a-bit harder I will start to hear the scratching sound bleed through the piano layer
Not entirely sure I understand what you’re trying to do, but here are some ideas…
The scratching is a continuously playing audio track? I suppose you could use an expander (many compressors can be set up to do that) with a sidechain, so that the amplitude of the piano track controls the amplitude of the scratching, exaggerating the dynamics. If you don’t have enough control in the compressor itself, you can chain multiple compressor instances to multiply the effect of the modulation.
Another option might be to do it all in a sampler, though the “free running” scratching voices might be hard to do with some samplers, and if you want only one layer of scratching to go with polyphonic piano, that complicates matters further. Not sure any sampler can do that without scripting. Layering the scratching samples with randomized start offset might be enough, though.