Can someone with the benefit of history explain please why the "Psychology & Neuroscience" is a single stack exchange site instead of two?
Psychology: the scientific study of the human mind and its functions, especially those affecting behaviour in a given context.
Neuroscience: a branch (such as neurophysiology) of the life sciences that deals with the anatomy, physiology, biochemistry, or molecular biology of nerves and nervous tissue and especially with their relation to behavior and learning.
merging them both together seems to me akin to merging say the software engineering and theoretical computer science stack exchanges.
Software Engineering: Q&A for professionals, academics, and students working within the systems development life cycle)
Theoretical Computer Science: Q&A for theoretical computer scientists and researchers in related fields)
looking over the full list of stack exchange sites I see no other cases of distinct subjects being merged together.