A 2-channel amplifier gives you outputs for two speakers. Of course, that doesn't mean you have to connect two subwoofers. You can hook up one, two, or maybe even four. To be safe, you just need to be aware of the impedance of your subs and the "load" capability of your amplifier. These factors will determine which wiring method you should use.
Typically, a car stereo amp "sees" a 4-ohm impedance. When we say an amplifier is stable down to 2 ohms, we're usually referring to the minimum impedance it can handle in stereo (2-channel) mode, not bridged (mono) mode. The lower the impedance (resistance or "load") an amplifier sees, the more power it produces, and the louder your music plays.
A common way to get a 2-ohm stable amp to produce the extra power it delivers at lower impedance is to wire your speakers in parallel. Remember — while series wiring always raises your impedance, parallel wiring always lowers it.
To wire your speakers in parallel, connect the positive (+) leads of both speakers to the amplifier's positive (+) terminal, and the negative (-) leads of both speakers to the amp's negative (-) terminal. You can increase your system's impact dramatically by hooking up two subwoofers (in parallel) to each of your amp's channels.
After you complete the wiring, break in your subwoofer properly. Play the sub at low volume for approximately 20 hours to condition it before you turn up the volume. This will improve the sub's performance and lifespan.