Though I agree with the detailed differentiation, still for the headline purpose a generic one that resonates with people easily isn't a bad choice
I don't agree. For that you need to distort the definition of what an organ is to comply the requirement for two separate organs with distinct origin, genetic, and biochemical traits, and different function and responsibilities to somehow remain classified as the same organ.
"Hence, we postulate the brain is a composite organ emanating from two lineage-restricted progenitors; these dual progenitors may be evolutionarily conserved across 550 million years from hemichordates to mammals."
"Human brain was two separate organs"
Then, both attention grabbig and true?
(if I'm reading parent poster correctly)