It actually already splits in the tileset, and the thing is that it seems like unless the window of the set itself is enlarged (which as far as I know isn't possible) I don't think there is physically any space where more could go...
I have never used RPG Maker. Will images at a lower resolution not help?
I'm relatively new to it myself, so not sure, I currently consider myself lucky if upon loading the image it's visible at all in the tileset...