Extra coils are usually only for convenience.
Amplifiers operate at certain resistances. This of resistance, current and voltage as a water hose. The size of the water hose is what is restricting the amount of current from passing through with the voltage being how fast it comes out. If you make the hose bigger(less resistance), more water(current) is able to come out but with less voltage.
Now, if you got the analogy above, know this. The circuitry on an amplifier is build to handle a certain impedance (another word for resistance). With less resistance comes more power, and an unfortunate byproduct of power is heat. So if the amplifier sees a load that is too low (which is totally dependant on the way you wire your speakers) then it could potentially damage the amplifier. An amp can, however, run at any impedance higher than what it is rated, because more resistance=less power=less heat. So what you should do is read the specs for your amplifier, then chose from that what type of subwoofer you should buy. If your amplifier puts out maximum power at 2ohms, then buy a dual 1ohm, dual 4ohm, single 2ohm, or any other driver with a VC config that allows a 2ohm final operation.