I have a rockford fosgate 12" sub. It says Impedance 2 Ohms but the sub has dual voice coils. So does that mean I sould still use 2 Ohms or should I use 4 Ohms because it has two voice coils.
That depends on what your amp can handle. with that sub you can go 1, 2, or 4 ohms 1 ohm wired in paralel, 2 ohms if you only wire one coil, or 4 ohms if you wire it series.
If you want the sub to last long, I would make sure you connect both voice coils. Most manufacturers include warnings in their owners manuals that specifically state that both coils must be hooked up. So you either have a one ohm sub or a four ohm sub.