Its really about how they are connected to the car from the factory that lets you know what type of impedance the amp is seeing normally -
Like for example this would keep the factory impedance of 8ohm (substitute 6ohm for 8ohm)
2 DVC drivers with Voice Coils in Series / Parallel
Connecting the two voice coils of each driver in series (+ to -) and the drivers themselves in parallel (+ to +, etc.) will result in the following impedances:
Dual-6 Ohm Subwoofers: 6 Ohms
Dual-4 Ohm Subwoofer: 4 Ohms
Dual-2 Ohm Subwoofer: 2 Ohms
Dual-1.5 Ohm Subwoofer: 1.5 Ohms