2 subwoofers and frequency response

The more sepakers you throw intoo a space, the more you need to spend time making sure the speakers sound good from the listening positions. If not, you'll run into all sorts of sonic compromises.
If you place the subs in corners, you'll DEFINILTEY need to EQ them very heavily to smooth out the sound. Also if you place half way between wall boundaries, for smoother response, you'll have to deal with phase issues if you have a large seating area.
Probably the most easy way of getting two subs (more dynamic headroom potential, lack of distortion, and solidity of frequencies covered) integrated easily into a room (forget decor considerations) is to place another sub right were tbe original sub -that's properly located for good sound already - is located! (i.e, stack on top or right next to each other (likely straddling a middle wall boundary).
My personal experience is that placing two subs in between (bellow a center channel) a front wall, where the main speakers is, is usually a fool proof way to integrate well, get phase right, smooth frequency response quite a bit. And the main L/C/R setup IS the most critical to get blended correctly with the subs/seating positions, as that's the main soundstage focus area, usually carying the most weight.
If you are the only one or two seating options in a room, you'll probably get the smothest fundamental response (adjust phase control accordingly) if you, yes, place one sub in middle of front wall, and one in middle of side wall. (I doubt most oculd put a sub half way up a wall to further smooth out bass modes in a room).
Bottom line you'll have to balance things well, consider strongly phase response between both subs ( you can easliy cancel out the bass between both subs if not careful, considering diffent seating positions), and integrate them with care to get the best out of things.
I still recommend placing two subs on front wall in center for easiest. But, yes, two can offer more better than one. But it all depends on execution and overall setup

Adding more woofers in a given acoustic space accomplishes many things - and introduces a challenge or two. Dual woofers allows for more dynamic range and efficiency in the bass - more power and surface area equals all the above, plus can help eliminate distoration, provide higher accuracy potential, and more. Getting another woofer in the system, allows you to lower the output of each - again, more efficiency.
Another common concept is dual woofers smoothing the bass response, by locating them in different possitions in relation to the bass modes and the seating possitions/speaker locations. This can be tricky to pull off with maximum effectiveness, however, in a multiple seating arrangment, where you sit proximally closer to one woofer vs the other, cause phase issues, and so forth.
Dual woofers can also be out of phase with each other, and relative to the rest of the speakers in the system.
Yes, Stereo Subwoofers also increases efficiency, and can help locate the bass and proper phase with each speaker with some added ease and effectiveness. (an EQ often helps here, regardless).
Whether to add another sub depends on lots of variables. Sure, in smaller/medium sized spaces, and in less ambitious systems, multiple subs might not makes sense. Still, to be certain, more can indeed be better in an "all out" system, where maximum impact, and system efficiency is demanded. I think the benefits outweighs the negatives - but ONLY as long as the rest of the system in wisely constructed and dialed in, and a there's a little knowledge involved with what's going on with the rest of the set up. Othewise, default to "keep it simple, stupid" is probably the wiser route - stay with the single
...in this case throw a smaller sub in a corner, and a bigger one right up in the middle of front wall between main speakers, and let today's advanced digital EQ's help you out with the rest.

Oh, didn't see your post on room size. I'd probably stick with one well placed sub in that smallish space. Two will get croweded aweful fast, unless you get real creative. Also, you have enough output from that single sub in that space. Also, the very lowest feq's won't be properly represented in that space anyway (could have gone with high output 10" woofer in that space, also). Bass will be on the thick and slow side on the bottom, as room won't absorb enough bass for a proper RT60. In rooms like this, I like to stay with woofer(s) that don't play down as low as larger, because natural boundary reinforcement and RT60 balance better this way. Otherwise you spend your time and technology tying to TAKE AWAY bass energy, and eqing out the large hump at the bottom from boundary re-inforcement, bass nodes, and lack of absorption. Better to have too little that's properly blended than too much that's compromised.
Yeah, stay with the single and EQ it out. My advice, anyway.