what's the main purpose of connecting two zeners facing each other?

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It's for over voltage transient suppression.

The breakdown voltage is equal to the Vf plus the reverse breakdown voltage.

You can buy transient suppressors which are just two zeners connected back-to-back. The advantage is that it can be used for AC.

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The usual application for this is for use in ac circuits to clamp an ac voltage equally on both positive and negative halves of each cycle.

 

bipolarity clamp ±(Vz + Vd)
It is also used across the capacitor in OPAMP integrator circuit to clamp the output to a know level and stop the OPAMP driving itself into saturation (and thus cause issue and delays if/when it is winding out of saturation)

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This back-back connection can also be used to clip the sinusoidal signal depending upon i/p and Vz of diode.This can be extended further to generate square wave.