hhhmmm unfortunately it's not as easy as saying "2 15 inch subs will run effectively off that amplifier". You have to look at the total wrms power that would be put out by the 2 15" subs you choose (so 600wrms for 2 of the JBL's (they call it 'nominal' which i believe is almost exactly the same as the wrms rating).
You then have to consider wiring configurations to suit the specific subwoofers and make sure that the amplifier is "x amount of ohms stable". An amplifier will USUALLY produce more power at a lower level of resistance (ohms).
Anyways, if you were to add another one of those JBL's you'd be best wiring it with the voice coils in series and the woofers in parallel for an overall 2 ohm resistance at the amplifier. Go to this page and select the details from the boxes and you'll see a diagram (choose 2 for no. of speakers and 2 ohm dual voice coil for the voice coil impedence):
http://www.the12volt.com/caraudio/woofer_configurations.asp
Problem is, you'd have 600wrms power handling from the 2 subwoofers and only 360 wrms power from the amplifier. So an answer to your question - yes it would work but you would only be getting a little over half the required power to the subwoofers as compared to what they're rated to handle. So if you had the gains set too high on the amplifier you could cook something or send it into protection mode. In essence, it would work, but wouldn't be providing the power 2 of those subs would deserve.
Hope this helps, cheers, Bryce.