2 subs can be a blessing and/or a curse. You think you had a hard time getting your existing setup tweaked? Add another sub into the mix... Don't get me wrong, another sub in the back of the room CAN make a huge difference but it's 10x the PITA factor to try to get it to blend with the mains and the other sub, stay in phase, not cause cancellation issues, etc.
On the other hand, IF you have the flexibility to place it where you need to, you can play games by inducing a peak in response from that sub at the seating position where the other parts of the system are causing a null, etc.
Not trying to scare you off - just get ready for a weekend of moving and tweaking. This is the time where an RTA is worth it's weight in gold.
Bryan