In this case it means the speaker respondes to two different signals at the same time. One voice coil for the left, and one for the right.
I don't really know how it works in the Emp Tek rock speakers, but I do know that dual voice coils are useful in messing with a sub's impedance. For example a 2 ohm dual voice coil speaker can have a 2 ohm impedance (just one voice coil used), a 4 ohm impedance (both voice coils wired together in series) and a 1 ohm impedance (both voice coils wired together in parallel) and these different scenarios will actually change many driver behavioural parameters.
Now a 1 ohm impedance sounds wild, but car audio amplifiers are often designed for this load. How well they do it I don't know, but it is what it is. The lower impedance gives them more sensitivity I think.
Speakers like these are rarely 4 Ohm and almost never 2 Ohm, so a 1 Ohm load is out of the question. These are 8 Ohms per voice coil and the reason is that for distributed audio, nobody is listening and saying "Oh, the separation and placement of the instruments is incredibly life-like". Distributed audio is what the name infers- it distributes the audio but the main goal isn't to have 3D stereo imaging, it's so both channels are heard without dead spots. You over-thought this by a mile.
Car audio uses 4 Ohm speakers because a car amplifier doesn't have a high voltage power source. Back, before power amplifiers for cars existed, the radio or tape deck was low powered because it was A) not a requirement since the music wasn't played at ear-splitting volumes, B) it was difficult to develop high power with discreet output transistors without high voltage power supply rails and C) if they did it, it would be too expensive and nobody would buy it. Common ground radios put out an honest 3.2 W/ch at anything close to low distortion and into a 4 Ohm load, 2.83V (used as the reference voltage for 1W into an 8 Ohm load) is 2 Watts. Unless a "vibrator" was used, power supply voltage higher than 14.4VDC was impossible unless the car's voltage regulator went bad, so that was as high as they went. Once it became much easier and less expensive to make a high powered amplifier, they became common and now, head unit amplifiers put out a lot more power than they ever did before.
The reason they can use 4 or 2 Ohm speakers in a car is because the battery is an extremely high current source. There's no way to deliver 100A of current from a 120VAC receptacle unless you want to see a lot of smoke and fire trucks.
