To effectively double the volume heard by the human ear, it would be necessary to increase the power output rating of the amplifier by a factor of 10. A more effective way to increase the volume from your system is to add additional speakers. Speakers move air, and the movement of air creates pressure in a room. This as commonly known as "sound pressure," and it is measured in decibels (dB).
Decibels are measured in a logarithmic fashion not unlike the Richter scale. If you are reading 103 dB on a meter and you increase the volume to 106 dB, you've effectively doubled the perceived volume. By adding speakers, you're moving more air, thereby increasing apparent volume.
There's an interesting phenomenon known as "coupling gain," which happens when like speaker cabinets are placed next to each other or on top of each other, causing them to "couple" and creating a situation where the whole becomes greater than the sum of the parts. You get a "free" 6 dB of gain when the cabinets are placed together. If you normally play at or around 100 dB and you place an identical cabinet on or next to the original, you will more than double the volume perceived by the human ear. Split the cabinets, and you'll hear the volume decrease.