To add to the previous comments, an autoformer such as the Zero inserted between an amp and a speaker will provide very different (and most likely much better) results than if a speaker driver or a speaker is inserted in series between them.
First, an autoformer will not consume a significant amount of the power that is being put out by the amp. While a 4 ohm load inserted in series with a 2 ohm load will consume two-thirds of that power. And a 2 ohm load inserted in series with a 2 ohm load will consume half of that power. (Note that I used the term “load,” rather than “speaker” or “speaker driver.” The impedance of a 2 ohm or 4 ohm speaker will certainly not be 2 or 4 ohms at many frequencies, and will not be purely resistive at most frequencies either. Which leads in to my next point).
Second, note Spatialking’s reference to using speakers that are identical to the ones you have. If they are not identical, most likely the frequency response provided by both speakers will be significantly altered by differences between how the impedance of each speaker varies as a function of frequency. That is because how the power provided by the amp at a given frequency divides up between the two speakers depends on their impedance at that frequency. If the two speakers do not have identical variations of their impedance over the frequency range the power provided by the amp will divide up differently at different frequencies.
Third, if your present speaker and a speaker that is added in series and is also producing sound are not equidistant from the listening position "comb filtering" effects could very well result, which would degrade the sound.
And on top of all that, of course, there would be the cost of adding a second pair of speakers.
The bottom line: Stay with the autoformers :-)
Regards,
-- Al