Getting a second sub will help even out the bass in the room. There can be and mostly some cancellation but, the end result is a more even room response. If your house is an open concept style, it may be hard to pressurize the room. What size room do you have? I could not view the pic's.
Klipsch RF 7 based HT 7.4, Pioneer SC 35, Acurus 200 Five, Dayton 18 Ultimxa Dual Sub Cab(2), Dayton 18 Ultimax Large Vented Sub Cab (2), on Berhinger I Nuke DPS amps, Samsung BDP F 7500, Asus/My Book Live HPC 4 TB
Yaquin VK 2100 amp, McIntosh XR 5 speakers, Samsung BDP F 7500