If you want to connect 2 speakers to your amplifier, the best way for you to do it would probably be in series. This is the safest way for your amplifier. As described below:
"Series Connection:
This will explain how the impedance changes when speakers are connected in series. In a series connection you simply connect the positive terminal of speaker 'A' to positive terminal of the amplifier. Then you connect the negative terminal of spkr 'A' to the positive terminal of speaker 'B'. Lastly connect the negative terminal of speaker 'B' to the negative terminal of the amplifier. If both of the speakers have an impedance of 4 ohms, the total impedance will be 8 ohms. In a series connection, you simply add the individual impedances. If there were three 4 ohm speakers in series, the total impedance will be 12 ohms."
As for the volume level Audioholics has this to say:
"So how does this equate to sound pressure levels?
Since we connected two identical speakers in series with our amplifier, each speaker only sees half the voltage drop across it thus as a result will see only 1/4 the power delivered to each speaker compared to a single speaker connected to our amplifier. The equivalent SPL now produced by each speaker is 6dB lower than if a single speaker were playing off the amplifier, for a combined overall -3dB drop. However, running two speakers effectively doubles the volume displacement compared with that of one speaker. Thus playback through the two drivers results in a 3dB gain. Adding this to the 3dB drop previously mentioned and the net overall sound pressure level will remain unchanged. Thus, playing two identical speakers connected in series off of a common amp (as opposed to playing just one speaker off that amplifier) results in no level drop, when compared to the single speaker case. This analysis, of course, ignores mutual coupling and any room-induced acoustical artifacts. However, if the speakers connected in series are not co-located and summing perfectly in the room, the net SPL would likely be up to -3dB lower than playing a single speaker off the same amplifier. The net SPL product in this case has a dependent relationship on distance between the speakers and frequencies they are destructively interfering in the room."
