When you connect two pairs of speakers to an amplifier, the combined impedance is halved. So, when two pairs of 8Ω (Ohm) speakers are connected, the overall impedance becomes 4Ω, even though the individual impedance for each speaker remains unchanged at 8Ω.
The CXA61 has a power output of 60W RMS per channel into 8 Ohms, increasing to 90W RMS into 4Ω.
This quoted amplifier power rating is per channel. If you attach 2 pairs of 8Ω speakers, the overall impedance halves to 4Ω, so you'll be sending 90W per channel. This will then split between the two speakers attached to that channel, sending 45W to each speaker.
For the CXA81 the same as above will apply but as the CXA81 outputs 80W RMS which is increased to 120W at 4Ω. This means you will get 60W for each speaker when two pairs are used with the CXA81.
It is not recommended to connect 2 pairs of 4 Ohm or 6 Ohm speakers to the CXA61/81, as the impedance is halved meaning the impedance will be 2 Ohms or 3 Ohms respectively. This could result in the amplifier being driven too hard and CAP5 protection being activated, or damage being done to the amplifier and/or speakers.