Two identical speakers in phase with each other will be 6dB louder than a single speaker. Every 10dB of level has the subjective equivalent of sounding twice as loud, so that's a significant increase in sound pressure level.
Every time you increase the sensitivity of a loudspeaker by 3dB, it is akin to doubling the effective wattage of your amplifier. Every time you double the wattage of your amplifier, you net another 3dB of sound pressure level.
Using these calculations, you can dial in exactly the pair of speakers that will be ideal for your situation. Or, you can just get a pair of Eminence Wizards lol.
You can read about the maths here if you like:
http://www.sengpielaudio.com/calculator-coherentsources.htm
So if you have a single 95dB sensitive speaker and you go with two 98dB sensitive speakers, your total increase in SPL will be 9 dB, or just about double the volume level.