In the hypothetical case of unilateral thoracic facet join, when may I choose to perform the ant-sup glide to the upper or to the lower vertebra at the transverse process? In order to stretch the capsule and open the join.

The angle (or slope) of the spinous processes of the cervical vertebrae and those of the thoracic vertebrae differ considerably. The segment to stabilize and the segment to mobilize depends on what you want achieve i.e. opening/gapping or closing/approximation of the vertebral facets.

Will does a great job at explanation with his video ”biomechanics of the spine and spinal mobilizations’. I highly recommend it as I believe that it addresses your questions.

Video