It sounds really good. I do have one problem, though, that being that Nightmare Cuphead is awfully quiet, and I guess BF, too. Thankfully, this is an easy fix. Other than this, there isn't any issue.
Heres an idea to make the chromatics of void not sound in the bg music Option 1:You could remove the parts where void sings and replace them in NC chromatic scales Option 2:Overlay the chromatics of NC but make it louder than Voids Chromatic scale Option 3:Heres the most easy method...Lower down the volume of voids chromatic scale Hope this helped!