FullSink18, ohms are values of resistance. If your amp can support a certain ohm value, then you must not show it a value lesser than this amount. Higher will work, but the power that it puts out will be reduced.
In your case, if your amp is rated 600 x 1 @ 2ohm (rms??), its most probably a D class bass amp (mono). You can show it any impedance (resistance) equal or greater than 2 ohms. So if you wanted to connect a single 4 ohm sub to it, then theoretically the amp would output roughly 1/2 its rated power. About 300 wrms.
If you connected a 2nd sub, again 4ohms, in parallel, which would yield a final impedance load of 2 ohms. Then your amp would be putting out its full 600 watts rms. And each sub would see 300wrms.
Read this :
http://www.lalena.com/audio/faq/wiring/
Everything clear?