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
