Whether you wire two speakers cabinets in series or parallel, it's never a good idea.
Two 8 ohm speakers in series is effectively 16 ohms that the amplifier sees. Less of a load.
Putting two speakers in series, effectively divides the maximum voltage you can develop across each speaker in half.
This doesn't unfortunately divide the power dissipated by each in half, but by a quarter of what the single speaker dissipated, because power is the square of the voltage divided by resistance.
Simply put, the volume will be a lot lower with two speakers in series and you'll have to turn it up a lot more to get the same volume. When you do this you are feeding more noise floor.
Distortion will also be added to the resulting sound because both series speakers can have a different impedance/frequency curve if they're not a matched set. This impedance imbalance will upset the voltage that would have dropped across each individual speaker if they were used alone. You will alter the response of the system.
Alternatively, If you hook them in parallel the resulting lower impedance load (now 4 ohms) will likely put more of a strain on your amplifier. The impedance spec of a speaker is frequency dependent and can be quite a bit lower than its stated nominal impedance, so the effective impedance in parallel can get quite low. This will show itself in the form of heat and possible thermal shutdown if the amp is pushed beyond its thermal threshold.
Even driven with independent amplifiers, placing sets of speakers side by side may cause unwanted distortion of the sound through cancellation.
Probably better to stick with one speaker. Most people seem to think that adding more speakers will somehow help. Generally more quantity does not equal more quality.
