Sometimes, adding a second subwoofer can smooth bass response throughout the room. This is due to strong acoustic standing waves in the room which are dependent on the basic room dimensions (height, length, and width) and the placement of the sub and primary listening area. With a single sub, it is possible to obtain strong bass at one spot, with very weak bass elsewhere in the room. You can hear this by carefully listening to bass as you move a few feet in any direction. If you have strong bass/weak bass problems, using a second sub in a different location may reduce the severity of the problem. The important thing to remember is to find what sounds best to you! Each room is different; experiment until you find the placement that produces the most pleasing bass to your ears.
Some people feel they can never have enough bass. So long as they are placed properly, multiple subwoofers typically produce more bass. It is important to note that unless the second subwoofer goes deeper than the first one, adding additional subwoofers will only raise the bass volume—it will not produce deeper base. You’ll need to experiment with different positions (as previously described) to find the best places for two or more subwoofers. Some people use one subwoofer for a certain frequency range and the second for another (such as the LFE channel in 5.1 recordings). Other options are to connect one subwoofer to the front channels and one to the rear channels, or one to the center channel and the other to the remaining channels.