One of the questions often asked when building a custom car audio system is to get two subwoofers or one. The answer is not a simple one. For most, a single subwoofer is the better choice, while for others dual subwoofers work better.
There are several things to take into consideration when making this choice. The amount of space you have to work with, the type of music you prefer, and the cost of the subwoofers as well as a dual subwoofer enclosure.
The advantage to running 2 smaller subwoofers is a sharper, louder sound with a better clarity. Sound wise there is not enough difference for most people to adopt dual subwoofers in their systems. But there is a difference. Part of how subwoofers work is by displacement of air. So a dual subwoofer enclosure with two 12 in subwoofers will be clearer and louder then a single 15 in subwoofer will. Due mainly to a greater surface of air displacement.
The only advantages to running dual subwoofers are loudness and clarity, but even that depends on the size and quality of the subwoofers. While two 12” subwoofers may sound better than one 15” sub, two 8” subwoofers will not.
Two subwoofers are more expensive than one. Though perhaps not by as much as you may think. A single good 15” subwoofer will cost around $500, while a set of 10” subwoofers in a like quality will cost around $600 with the added clarity of sound. If space is your concern, a dual subwoofer enclosure often takes the same amount of room as a single subwoofer enclosure does.