@drew And it misses something else that you mentioned (in comments, by the way, which is the wrong place for something so fundamental and crucial), "... but more importantly the phase difference must be extremely stable over time." Please put that comment into your question. And elaborate a little on it, as well. It's not something one leaves on the table and ignores while coming up with an approach. It's something that you place front and center at the start. In fact, it likely determines much about the resulting design.
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