Solid state amps are less sensitive to impedance than tube amps, because they generally do not pipe their output through a transformer.
So if the internal speaker is 8 ohms and the spare out is wired in series, and you add another 8 ohm speaker, the amp will see 16 ohms as a load.
If the amp makes, say 65 watts with an 8 ohm load, a parallel speaker would reduce the load (4 ohms) and ask the amp for more output (100+) while a series speaker would increase the impedance of the load (16) and cause the amp to make less output (~30 watts).
Though, a doubling of speaker surface area would increase the apparent loudness by about double, so you get almost the same volume (or just slightly more depending on the efficiency of the speakers) but you will get a much wider sound field and so, better coverage.
Roadie & Reviewer