Yes, when two speakers of the same impedance are connected in series the resulting impedance is the sum of their impedances; when connected in parallel the impedance is half of the individual speaker impedance(the calculation is more complex when more than two speakers, or those having different impedances are involved). Of course simply using the nominal rated impedances doesn't give a complete picture of the result, since the impedance can vary widely at different frequencies. For example, my M22s drop to about 3 ohms in one frequency range, which is lower than the M80s ever get.
