Getting 2 subs

And you should try positioning any subs asymmetrically. 

agree that asymmetric is the key ... 

my experience with large subs is that the frequency is to low and for me to hear close up , so is it ok to get 2 different size subs to cover a wider spectrum of bass or get 2 matching subs?
 

You’ve described  my successful approach to increasing the low frequency SPL in a sound treated 24 x 18 x 8 ft listening area. The front wall has 2 corner placed Rythmik subs supported by a pair of smaller REL’s as one on each sub wall on each side wall. My speakers are Voxativ single drivers — fast and midrange rich, but relatively bass deficient.

Just added a pair of Rel 212SX to pair with my Borresen's and they sound amazing. Room Size is 18 X 24 and they are more than enough! 

As long as both subs are sealed, you shouldn’t have any issues integrating them. Don’t mix a sealed and ported sub in your configuration.

If you don’t want your neighbor to get thumped and just want overall clean immersion,

a) For starters, use the Harman room mode calc and position yourself out of big nulls.

(As a "general solution", i.e., it’s hard to say much without measurements or knowing anything about your room dimensions. Sbir is an issue with subs as well..that dudes don’t think about often enough.)

b) Try setting your subs a bit nearfield i.e. subs behind your couch up against each side wall as long as your room’s not too wide. If the subs get too close to you, it will shake your seat/couch like tactile sub bass drivers and become distracting. If the sub placement line connecting your 2 side walls is about 2 to 3 ft behind your couch, it could put you in a immersive womb of bass without thumping your neighbor’s skull.

(There are many goofy thought processes on AV forums about pressurizing entire rooms with a platoon of subs, which will only pressurize the neighbor and your wife’s innards and gizzards. Wife could run off with the neighbor and start collecting alimony payments from ya...not good)