Part of this whole discussion is the confounding of the concepts of light and darkness in both realms of their individual existence. Then you have introduced a new concept - "mingling" of light and darkness. Rather I would say that spiritual light and darkness is more akin to light and darkness in a room. The room I am presently sitting in has areas of darkness which are (for instance) under the table where the light is not so good or in the closet where there is no light. It would seem that though they can exist together but they are mutually exclusive.You're right but they can be right next to each other.
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In the life of any believer that has lust, lies, immorality, theft, jealousy, envy, gossip, gluttony, or selfishness in any part of his personal temple...then those parts have darkness and the darkness will stay there until the sins are confessed. "Give no place (Greek = Topos, 'room, chamber') to the devil.' Eph. 4:27. Think about it: why would the Holy Spirit move on Paul to pen those words warning against the mingling of light and darkness in the soul if it was not possible in the first place.
James 1:17 Every good gift and every perfect gift is from above, and cometh down from the Father of lights, with whom is no variableness, neither shadow of turning.
On the other hand if you mean a kind of mingling of light and darkness such as in the dawn or sunset then yes I would agree when it comes to soul or essential being of mankind (but not God of course).
HankD