There are many to get the flattest possible bass response in your listening room. One is not to have a room, of course. Cos walls reflect sound waves and these then build on each other to reinforce themselves leading to several one-note bass issues. Several? Yes, because the walls on your left and right have one "standing wave", one between your front and back walls, and another one between the floor and ceiling (which is why sloping ceilings are better from a hi-fi perspective). And there can be multiple smaller ones too.
The other way to get a flat bass response is to install bass traps. These slow down the bass passing through them and bouncing back (usually off the corners), thus reducing the chance of a wave of sound building up. If you can, do so. Bass traps are great.
The last way is to utilize each of these waves to help even out the bass response in the room. Every standing wave also has a dip, not just a peak. So what if the second standing wave has it's peak where the other standing wave is a dip?
That's how multiple subs help.
They even out these peaks and dips. It also means they need to be in different locations in a room. Both in corners will accentuate the problem. The second needs to be in a different position so it's peaks and dips are not adding to the problem!
I suspect a third or even a fourth sub will help smooth the bass out even more.
My own system has 2 powered subs. One in an alcove together with the main speakers (yes not the best place for loudspeakers, never mind a sub!). And the other in almost the centre of the room under a sofa. They have certainly evened out the bass in the room. You can walk the entire length and breadth of my open plan house and not have spots where the bass is crazy loud, or missing. Something which is always the case in a listening area with a single or no subs. Plus small subs have a better transient control, ie. tighter bass.
Give multiple (small) subs scattered throughout a room a try. You'll be glad you did.