Two PCis will provide you with more sheer output than just a single one. I built a clone of my 20-39 PCi last year w/an identical driver and it definitely helps not only in output, but as Sheep mentioned, with proper placement and calibration, it can help even out the response of your room.
Are you looking to improve one/both of these or a difference in sound quality? Two as opposed to one may also help from an SQ perspective if you're taxing your existing PCi... -TD