This is a nice question that many people in audio try to address. In order to have a complete answer though, one must take into account things such as, amplifier current capabilities, loudspeaker sensitivity, damping factor loudspeaker power handling capability and possibly a whole lot more.
An (audio) amplifier is an electrical device, most often, made to provide as much current as possible while at the same time maintaining voltage unchanged compared to its input times a constant factor (the gain of the amplifier). You may ask, why current is the sought quantity. This is because the force that will move the speaker cone, in the simplest (idealised) formulation is F = B L I (I would love some MathJax/LaTeX formating here), where F is the force, B is the magnetic flux (coming from the speaker's magnet), L the length of the coil (conductor immersed in the magnetic flux) and I is the current flowing through the speaker coil (coming from the amplifier).
As you very well stated, it may be desirable to cable 2 instead of 1 speakers in a single amplifier channel. To start easy let's assume you have two 8 Ohm speakers. In order to connect them to an amplifier channel that goes down to 4 Ohm you'd have to connect them in parallel resulting in 4 Ohm in total. Numbers look absolutely great!
Now, let's try to find out what the current demand will be for the amplifier. Guess what, it will be the same as the current that will have to produce if a single 4 ohm speaker is connected. Of course, this makes sense if you use Ohm's law.
Now, the problem arises... If you provide I (current) to two speakers, you'll get I/2 at each speaker (current divider). Thus, according to our (extremely simplified) analysis above, each speaker's cone will feel a two fold weaker force. Considering all parts of the speaker system linear (which are definitely not), the resulting pressure will be half of the pressure that would have been produced by a single loudspeaker fed I Amps of current. Then you'll say that if you now combine both speakers you'll get the same pressure output as in the single loudspeaker case. This is, in numbers, absolutely true, but I urge you to try to combine coherently two loudspeakers throughout the full spectrum (hint: if perfectly coherent summation was possible for an extended spatial area a lot of problems in the touring industry would have been solved!).
There's more to it
Audio amplifiers have a load impedance rating for some (good) reason. Since those real amps are not idealised voltage sources, which keep the voltage constant no matter what the demand in current is, they will exhibit voltage drops and other non-linear behaviour when pushed too hard. Thus, you could possibly use an amplifier to drive a 2 Ohm load but you could very well get bad results (distortion, dynamic range reduction) or even no audible artifacts (this is rarely the case but can happen if you don't crank up the volume).
The behaviour you will get depends on many complex parameters, including but not limited to:
- The topology of the amplifier.
- The design of the amplifier.
- The way measurements for the load rating were done and whether the given value corresponds to the actual value or it is "quantised" to the closest standardised one.
- The impedance of your loudspeaker. I urge you to look for an impedance graph of a real loudspeaker to see that this is not even close to a constant curve and it is not a good practice to represent it with a single number!
- The stability of the provided values. For example, in passive crossovers the resistance and other component values may very well change due to extensive heat (resulting from high currents passing through them). Here you can also see Power Compression.
- The material to be reproduced as well as the volume.
- A whole lot of non-linear phenomena such as (but not limited to): Intermodulation Distortion, Harmonic Distortion and Damping Factor which can very well increase the demand on power needs from the amplifier.
The solution that many manufacturers have provided at this moment is to "custom-tailor" the amplifiers to be used with their loudspeakers. This is most prominent in the touring and "high-power demanding" field of sound reproduction where integrated solutions with amplification and control DSPs are provided or even self-powered speakers. There are hi-end as well as monitoring system manufacturers that also provide similar solutions.