Having two objects Y axis parallel rotating around local Z axis.

Hi there,
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I have been trying to rotate objects around their own Z axis until their Y axis became parallel. But I cannot figure out any way to do this.
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I have try with the difference between quaternions (probably something rather than difference works?), and with lookAt (which does not help cause makes the objects to lose their axis Z orientation).
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Is there any possible way to achieve this? Any hint would be very appreciate.
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