When you connect two pairs of speakers to an amplifier, the combined impedance is actually halved. So, when two pairs of 8 Ohm speakers are connected, the overall impedance becomes 4 Ohms, even though the individual impedance for each speaker is still 8 Ohms.

The CXA60 has a power output of 60W RMS per channel into 8 Ohms, increasing to 90W RMS into 4 Ohms

This quoted amplifier power rating is per channel. If you attach 2 pairs of 8 ohm speakers, the overall impedance halves to 4 ohms, so you'll be sending 90W per channel. This is of course then split between the two speakers attached to that channel, sending 45W to each speaker.

In other words, if you attach 2 pairs of 8 ohm speakers per channel - (let's say the left channel) then the left channel will be outputting 90W... so each 8 ohm speaker will be getting 45W.