Subwoofers are speakers designed to channel the bass sounds in a speaker system. While subwoofers are not necessary for a speaker system, they do provide an added component for sound systems and improve overall quality. A subwoofer will be able to produce lower bass sounds than the other speakers in a multi-channel system, allowing you to experience a wider range of possible sound frequency. For large systems, more than one subwoofer can used for maximum sound performance.

Bridging

Bridging is connecting more than one type of a certain speaker to a mono amplifier that is designed to only channel one version of that speaker. For example, hooking up two subwoofers to a mono amplifier intended for only one subwoofer speaker system. This is a good idea if you want to minimize the overall impedance of the subwoofer speakers. Speakers have a natural impedance (signified as ohms) that they produce, the resistance of their internal components and wires to the electrical current they use to receive sound data from the amplifier. The higher this resistance (the more ohms), the more the current will struggle passing to and from the amplifier, and the more distorted it will become. This leads to worse sound quality and an inferior reproduction of sound as it was meant to be heard. Bridging can become complicated in two channel, stereo components, but fortunately bass is almost always communicated in mono systems, which allows you to bridge subwoofers more easily. The important part is to make sure that subwoofers are similar enough for the impedance manipulating to make a noticeable difference. For instance, two subwoofers with an impedance of four ohms will actually only have an impedance of two ohms when linked to a mono amplifier.

Multiple Subwoofers

To bridge the subwoofers, look at the amplifier and locate the subwoofer terminals. Even on stereo amplifiers the subwoofer ports will likely be mono, but amps still often have two or more terminals. This is simply to allow for large-gauge wire, and inside the amp all terminals meet at the same places, negatives joining to negatives and positives to positives. Hook up the subwoofers in parallel fashion, positive to positive and negative to negative (for dual voice coil systems, series/parallel configurations are required). If you do have a stereo system with multiple terminal for subwoofers, you can choose to connect a single subwoofer to both sides of the stereo system to reduce impedance. If you want to bridge more than one subwoofer on a stereo amp system, you will need to connect them both to the same ports, two positives to the positive connection on one channel, and both negatives to the negative terminal on the other channel. This will bring your amplifier load down to one ohm, which can damage amps unless they are rated for a one ohm load, so make sure you consult system information before you begin.

Source:

Crutchfield: Subwoofers Frequently Asked Questions

DLS: Connection of Subwoofers in Bridge Mode to DSL Stereo Amps

Youtube: Car Audio : How to Bridge 2 Subwoofers Together

More Information:

Fixya.com: Want to Bridge My Subs