The second speakers would be in the same room. The idea is to balance the voice of the first set, which are what some call "bright" or "analytical", with the second set that are "laid back" or "mellow".I would not recommend that kind of approach, even apart from the impedance and sensitivity issues that have been cited. What will happen is that the sounds from the two sets of speakers will not arrive at your ears at precisely the same time, resulting in what are called comb filtering effects. Which will most likely result in a significant loss in clarity, and a general "cloudiness" to the sound.
Also, a further point about series connection, which was properly defined in the post by Tony (Tls49): If speakers that are different models are connected in series the differences in how their impedances vary as a function of frequency are likely to result in significant frequency response irregularities in both speakers. The reason being that the voltage applied by the amplifier to the series-connected speakers will divide up between the two speakers differently at different frequencies.
And regarding parallel connection, keep in mind that the 2.4 ohm combined impedance of the two speakers is based on their **nominal** impedances of 4 and 6 ohms, and is likely to be even less than that at some frequencies.
Regards,
-- Al