Depends on if you want the ease of getting it in and out or not. If you do then you need the single, and tbh, 1 sub is good enough to provide a nice sound experience, but if you want it louder and bassier you'd wanna go for the two. However that being said, you need to make sure the amp is powerful enough to do both and also make sure you front speakers are uprated otherwise the bass will most likely drown them out.