This is a really important topic, and unfortunately most people don't have a very good understanding of resistance. Impedance is resistance, and is measured in ohms. Less resistance means less ohms, and higher is more. You need to be careful when working with speakers with low resistance. To visualize, no resistance, or 0 ohms, is the same as taking your speaker cables and touching them together. Commonly referred to as a dead short. So, with a 2 ohm load, you're 2 ohm's away from a dead short. That's why you have to be careful. Where most go wrong, is they see this as a good thing. They see an amp that is rated for 2 ohms has more watts available, so they get a 2 ohm speaker. Understand that while you can get more power by doing this on paper, its very hard on your amp. If you can do the same thing with a 4 ohm load and less watts, its a much better choice. Higher resistance means the amp has to work less, and usually sounds much better because its not straining.
There's also a couple of other concepts that are important to know. When you are looking at a 2 or 4 ohm speaker, keep in mind those are just averages. When in use, the speakers resistance will vary. At any given time it can possibly be a 7 ohm load, and at others, a 1 ohm load. Frequency is what determines this. High notes are more resistant, and ohm ratings will be higher at those frequencies. A lower note, means less resistence. Manufacturers often fail to accurately disclose this type of information. You need 2 numbers. One is the average like we've been talking about (2 ohms, 4 ohms.....). But the more important number is the lowest it will drop. It looks something like: 4 ohms nom, 2.5 ohms min. 4 is the average and 2.5 is the lowest it ever drops. The industry is doing a much better job with these specs in home audio, but car audio needs to catch up.
So, now applying this to your situation, you're looking at a 2 voice coil sub. As I'm sure you already know, this allows you to use 2 channels of an amp to power the speaker. The choice you need to make is, wire the connection in series or parallel. Series will create a 1 ohm load and parallel will create a 4 ohm load. Unless you have some very expensive, high end amps that can be put through hell, wire the sub in parallel and you should have no problems at all.