Another poster, wish I could remember who, noticed that...
In yet another scene with Ennis confronting feelings he doesn't know what to do with, in the bus station...
in the background there are two men, one in a black hat and one in a white, buying tickets to travel apparently together. The poster's interpretation was that this was a reminder of what Jack wanted for the two of them, being together, moving on somewhere, in public, together.
And I think maybe it shows what Ennis is coming to have in the back of his mind, since he'd ended things with Cassie, maybe he was getting ready to give something more to Jack, and it's already too late.