To make the divider of an NSSplitView really thin, i.e. 1px, it does not suffice to set the divider type to “Thin.” You have to set the border type of both embedded views, assuming that they are both subclasses of NSScrollView, to NSNoBorder. This means in Xcode
By default NSBezelBorder, the third button in the picture, is selected which results in
By having no borders, you get
which looks much better.