Getting 2 subs

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)

I’d think if another SB4000 isn’t in the budget an SB3000 shouldn’t be too hard to seamlessly incorporate into the mix between placement and adjustments so I wouldn’t sweat it. 

I have a pair of Matrix 802 happily assisted by a pair of SVS SB3000 with the preamp set for a 40Hz crossover. Along with an old Bryston 3B-NRB, I have had occasion to replay certain tracks to see if I’ve just been through an earthquake. The earthquake via SB3k was provided by the last 30 seconds of "Another Tune Like Not Fade Away" by Solex. The last two minutes of "Ontario Plates" by Do Make Say Think is another track that I thought was an earthquake.

I've experienced actual earthquakes, too many to remember, & the SB3k pair has fooled me a few times.

The SVS share a dedicated 20amp circuit and the Bryston has its own 20amp circuit (makes a huge difference). The SVS footies are a must unless you have a concrete floor.

I think you are going about this in a way where you still won’t be satisfied.  I would suggest utilizing 6 subwoofers stacking 3 on each side.  If I were you, I would either use JL Audio Fathom 110’s, 112’s or 113’s.  Another option is REL’s 6 stackable subs. You are missing the frequency at certain height levels which in turn makes you feel like you are missing certain frequencies and dynamics that should pair with your speakers.  You can use the room correction to eliminate the negative waves and vibrations of your room to get the exact bass frequencies you are looking for.  Each subwoofer stacked will be set at a different crossover point.  For me, I was glad I went this route.  It made a world of a difference. Hope this helps 

"I would suggest utilizing 6 subwoofers stacking 3 on each side."

I would suggest 10 subs.

Plus 1 in the bathroom.