As SMB said, you can create a virtual sub by having it at the intersection of two other subs ( say along two opposing walls). You may need to tweak phase and equalize with a parametric equalizer- only subtract. Do not add.
Search here and the web for multi-subs.
Here is some wisdom
Typically 3 to 4 subs helps get you there. They do not all have to be big monsters playing deep and shaking the walls. They just need to cover from the lower modal frequency that your room can support up to the Schroeder frequency (200-300Hz). Best placement may lead you to non floor locations and along different walls. Corners will stimulate room modes and lead to boomyness.
Then you also need acoustical treatments.
Most typical around here is to fill the corners with super chunk bass traps. OC703 stacked floor to ceiling with a 24" to 34" faced diagonally into the room.
More recently people are building bass traps with regular insulation - "pink fluffy stuff". Insulation deeper than 12" has the best absorption coefficient, at the expense of size/depth. Costs are low too. Just need to have a design that prevents compression of the insulation.
You can also find the peaks along the boundary of the room and surgically place low frequency absorption to broaden/lower the Q of the peak and help fill in the valleys of the response.