try reversing the normals on one of the pieces
if you've been duplicating and then using a negative scale on the object then frozen the transformation then that's what you need to do.
if you go Lighting > Two Sided Lighting and untick that then you'll see the faces become black on one side and coloured on the other.
this shows you the direction the face is pointing (the normal) without cluttering up the screen with those green lines
that's a "Ch" pronounced as a "K"
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