I figured a request like that would be made on derpibooru, but I don't see why it matters here on ponerpics.
Did the artist make that request to ponerpics? If not, there is no reason to keep them separated, especially since the sfw tag does not mention the nsfw tag existing. It convolutes things for no reason.
Might as well merge so people that search for that artist can actually see all of their content. Let the users decide if they want to see nsfw content by using rating tags instead of separating them.