A voice coil is basically a coil of wire that the powered signal from your amplifiers goes through. A dual voice coil speaker has its coil split up into 2 parts. The purpose of this is to give you more options on installing it. For example, if a sub has two 4ohm voice coils, you can wire it in paralled for a 2 ohm load (as seen by your amp) or in series for a 8 ohm load. If you want to get the most power out of the amp safely, get 2 single 4 ohm voice coil subs and wire them in parallel to achieve a load of 2 ohms. In this senario, each sub will get 250 watts RMS.