If I understand it in the right way, you may create the black shape in front of the cyan shape and give it a white stroke that becomes the white band (you may place it behind the fill), or you may Object>Path>Offset Path with the band width as the offset, then change the offset path colour to white.
I don't want the white border. I separated them for the purpose of this post. I want the blue path that runs along the black to align and shape EVENLY to the black one and then combine them but remain two separate objects.
If I understand it right this time, you may:
1) Create the combined blue + black shape,
2) Create a path where you want to separate them (the border of the black shape), extending past the combined shape,
3) With only the path selected, Object>Path>Divide Objects Below.