I’m a bit confused by the wording of this post? Is the problem that you have two .stl files that you want to edit and munge together into a new object? If that’s the case, then as @AmazingAwesomator@lemmy.world you can import them both into openscad, subtract what you don’t need or intersect what you do and place them into a new part for export. You can also do this in prusa slic3r if openscad is being buggy which it sometimes is with stl imports, but it’s a massive pain in the arse