The output of the top amplifier feeds the negative input of the bottom amplifier so their output voltage swings are doubled. With a +/-12V power supply The top amplifier output swings to a peak of +9V when the output of the bottom amplifier swings to a peak of -9V. Then the 8 ohm speaker gets 20W RMS just before clipping. When the supply is +/-16V and the distortion is very high then the output is 34W.
Bcos I can't see a direct connection. When I look at them, positive and negative input of second amplifier is connected to neutral. One with a 22k resistor. I'm not a professional. So not good with circuits.