Not having a single sub and been using dual, for the last 20 years or so. Honestly, I cannot tell you since your area is double than mine.How does it compare to a single FV18?
I have a single one in my living room open floor plan, about 3.3k cubed feet. I wonder if a second one would be much different. I positioned it in the corner within a few feet of the MLP.
First you should ask your self those few questions;
-How does-it sound to you with a single FV18?
-Do you feel like you can achieve your listening level easily?
If it sound good, and satisfied to be able your listening level. Than your single sub might be all that you need.
That said, having dual subs do have some advantage. The first one, both subs will play at lower level to achieve what one sub do. Therefore can achieve higher playing level with less stress on each subs, resulting in a better sound. Second one, it can smooth out the frequencies for more sitting area not just your main MLP as one spot. Another thing to keep in mind, when going dual subs. It is always best practice to have two subs that are the same. Since they will be much easy to integrate together, due that different subs are harder to integrate because of phase issues.
To learn more is a very good Guide on sub/s set-up;
Guide to Subwoofer Calibration and Bass Preferences
VIII-C: Selecting Single Versus Multiple Subwoofers: