the second driver gives you double the cone area and that leads directly to +3dB of low bass.
The second driver also needs power. If you double the power fed to the drivers that leads to a further 3dB of output over the whole range of driver frequencies. 6dB of extra output is not easy to find.
But you have a choice- fit both drivers to a common cabinet where they share the coupled volume or fit the two drivers to separated volumes inside the same cabinet or fit one driver to it's own cabinet and double up on the cabinets.
