We don’t sit facing the wall because zazen is an individual practice; it isn’t.
We are always practicing together. Not practicing, together. Practicing Together.
We sit facing the wall because we are sitting with all beings.
If we were facing each other we will be sitting with these beings, not necessarily all beings.
And when we sit, one more person sits with us. You could call this person Vast Compassion Space.
It is as if the door of the tiny room of the Self is unlocked, and the prisoners there are released into this vast space, to express, to change, to live, to go.
Were this person not to appear, the door would remain closed. Each prisoner would remain locked into their repetitive forms and gestures.
You could also call this person Faith Mind.