I agree with Mofi and Elizabeth. I would add that putting one speaker on each channel on the 4 ohm tap and the other on the 8 ohm tap will actually draw more current and power from the amp, and be more stressful on the amp, then putting both of them on the 4 ohm tap. The taps are not independent of each other; they just come from different points on the same transformer winding. And the 8 ohm tap will provide a higher output voltage for a given input voltage to the amplifier, causing it to deliver more power into a given speaker impedance if connected to the 8 ohm tap than if connected to the 4 ohm tap.
So I would suggest connecting both speakers on each channel to the 4 ohm tap. It may also be worthwhile comparing sonics with both speakers connected to the 2 ohm tap, because undoubtedly the impedance of the speakers dips down considerably below 8 ohms at some frequencies.
Series connection can sometimes be a viable alternative, as Yogiboy suggested, but I suspect not in this case. Since the two models are not identical, chances are the sonic results would not be very good, because the frequency response of one speaker would be thrown off by differences between its impedance variations as a function of frequency and those of the speaker it is in series with. Also, since the series connected speakers are nominally 8 ohms, you would be connecting a nominal total of 16 ohms to the 8 ohm tap, that being the highest available tap on this amplifier. That would result in the maximum amount of power the amp would be capable of delivering to the two speakers combined being greatly reduced perhaps cut in half.