Duality is what it says. Dual is two. A duality is something in two; there are two parts, perhaps of one whole, but if os one whole, the sub-parts must be exclusive. When light can be said to maintain the properties of both wave and particle; this is duality. Duality focuses on the twoness of one thing. Duality is an expression of parts. Dichotomy is an expression of distinct entireties, Two separate whole things even if they are related, maybe even contingent in some ways, but remain apart in other ways, under other analyses–this is dichotomy.
Dichotomy is the contrast between two things that are either different, as in distinct, or related in some way, yet maintain a distinction that is contrastive. Delineating contrast, thus revealing dichotomy, might have the effect placing two things under analysis in juxtaposition. Yes, two things represented as being opposed or distinct is dichotomy. And here “representation” is key. How things are presented again in the process of comparison is also part of what appears to be duality and what seems to be dichotomy–and here comparison is used in the fullest sense, that is, predicating contrast, for any comparison that does not contest is a limited and restricted way to compare. The language we choose is implicit in the representation of dichotomy and the representation of duality.
Can two parts of duality be made to appear dichotomous? I imagine this might be possible–probable, even. For Americans, mind and soul are a dichotomy; they are two separate things, perhaps related, perhaps cross-functional, but separate and distinct. For the French, they are a duality. Mind and soul share one word ad therefore have different connotations. In English, mind and soul have separate categories of inclusion and therefore remain, under some analyses, mutually exclusive. In other analyses, mind and soul in English language explications might share overlap, as one could express using one of those Venn diagrams teachers in Middle Schools across the United States are so in favor of using to show contrastive states and mutual states, states where conditions for one or the other thing are separate and where they might overlap or share mutability.
Mind and body in most understandings are a duality; so much more the suffering of a person if they are in dichotomy. The twoness of each is distinct from the other. It is online this twoness that they share any likeness, but it is not enough for them to share any synonymy. There is no synonymy for the two. In the sense that mind and soul share categorically defining criteria, they are in duality? Where they are separate, mutually distinct, they are dichotomous? The answers to these questions lies with the process of analysis, the analysis itself being the lens of refraction, the boundaries of representation. Nonetheless, in the French language, mind and soul are a duality, there mutual category of inclusion announces this; any distinction between the two is expressed by the one word having different connotations in distinct contexts of meaning. In the English language, the two ideas begin in separate categories and come together at certain or less than certain points where we can express likeness, similarity, something mutual and perhaps even reciprocal between them.