a bit late chiming in, but, this is not a naive question. the definition of organ precedes developmental genetics, as a collection of tissues composing a structural unit, fullfilling a distinct function. its a sloppy definition in the contemporary, but what it does do is provide definition with respect to organizational level.

the gist is that an organ is localized structure you can point towards, where as a diffused system, such as nervous system , requires a sweep of the hand, as it is distrubuted over the volume of the organism.

the "common function" is the floor of the definition.

what is clear from embryological study is that neural tissue is derived from communication between two embryonic cell plates, and the result is modified based on spatially based inductive signal consequences for gene regulation.

these are not two separately derived organs later fusing to one mass, these are different regions of the same mass having different consequences of inductive signal.