Another little mirror:
Within the scene that centers the film (Ennis comes home after the night in the Siesta, packs, leaves for the mountains). Early in the the scene, we see Alma (and only Alma) looking out of the window, at the end of the scene, we see her looking out of the window again.
The first time, Ennis has just been coming, only she's looking out of the window (one person) and sees Jack alone outside (one person). The second time, Ennis is leaving, she's to be seen with Junior (two persone) and she sees Jack together with Ennis (two persons).