In an ideal world, when you halve the impedance (load) you double the power, so it would be 1200 watts. Now, you lose some to heat, inefficiency etc so really power never doubles. With the JBL, being a class D amp, output is regulated at 1 ohm, so the 600.1 puts out the same power at 1 ohm as at 2.
short answer, it's 600 watts at 1 ohm as well as at 2 ohms. The same holds true for the 150.1, 300.1, and 1200.1, where power is the same at both loads. This is a feature of the BP series mono amps.