Yes. If you have a solid state amp (which puts out more power into lower impedance), then it will be a LOT louder. With a tube amp, it will be 'somewhat louder', since tube amps don't vary their output when cabs are added and impedance is lowered.
With a solid state head, you have more power, more speaker cones which move more air, and a 'coupling' effect that results in a synergy effect between the two cabs, resulting in more low end and volume. Put all three of these things together, and the difference is HUGE.
However, it will not be 'twice as loud'.