Is One Subwoofer system Good? Are Two Better?
If your room has typical dimensions--20 x 14 x 8 ft., about 2,100 cu. ft.--one well-designed subwoofer system with an internal amplifier equal in size to the full output of your receiver (full power for 1 channel + 1/8 power x the number of other channels) and a 10-inch or 12-inch driver should deliver solid deep bass extension and ample output for music and movie soundtracks.
On the other hand, if the room is larger than usual (4,000 to 8,000 cu. ft. or bigger) or has a vaulted or cathedral ceiling, you should definitely consider running an extra subwoofer system. Big rooms, especially the "great rooms" so common in many suburban homes, really devour deep bass, so two subs will generate enough sound pressure to fill the place. They'll also give you smoother distribution of extended bass over several different listening locations.
There is no specific need for two subwoofers to be identical in terms of brand name or physical size, but don't use a small, cheap sub with a large, good one.
The subwoofer system can go just about anywhere on the floor, but corners will give you the greatest enhancement of deep bass, at the risk of it sounding boomy. Moving a subwoofer or a floor-standing full-range speaker away from any intersecting room boundary will reduce the tendency to boom or to have too much bass. In either case, you will have to experiment to achieve smooth and extended deep bass in your preferred listening location. Bass output will vary in different spots in the room as a function of the room's dimensions, so aim for good bass extension in preferred seating locations. You can't satisfy everyone in the room, although adding a second subwoofer will help smooth out the bass for other listening locations. for more info. please read how to optimize subwoofer level.
Note: If you like your music or soundtracks really loud and deep, go for two subs. If you have a huge room and you want really loud sound and deep bass, then look at physically larger subwoofer system with bigger amplifiers.