There is little difference in the throw between a Cubo Sub and Cubo 18, though Cubo Sub is slightly better in that department. Cubo Sub will drop lower (usable from 30 Hz and up, for that very lowww-sound), Cubo 18 is more allround from 40 Hz and up but more compact.
@T.geessounds: It doesn't seem to mention any of the Thiel/ Small parameters anywhere. For a 4" VC, 120 oz. magnet 18" driver, I expect it to work but absence of T/S-parameters is usually not a very good sign.
Reducing the width of Cubo Sub like that will raise the lower usable frequency up by 3 - 4 Hz, making it a more or less 34 Hz and up cabinet.
@Skillz: You can mix them (on the crossover) but in that case I would prefer physically seperating them. Say the Cubo 15 left and right and Cubo Sub in a centerstack. If you're gonna mix them in a single stack, I don't necessarly expect trouble but since there are so many variables you never know for sure. If you're going to use the same high pass for both I would make it as high as possible and steep.