Gustavo surely had his reasons for naming his piece as he did. I think, as you intimate, the title should be approached poetically rather than literally. Wings are a bird's instrument of flight, and this evokes associations of freedom, exhilaration, and possibility. Birds are creatures of the air, and are borne up by the wind--the very element which is most closely associated with Jack. I think 'The Wings' is tied in with Ennis's feelings about Jack, and everything positive that Jack represents for him. I see it more as Ennis's theme than the theme of both men together, though it is Ennis's theme
specifically in regard to how Jack has touched his life.
Finally, one might see 'The Wings' as intimating the flight that Ennis, and we all eventually, will take in pursuit of him (and those others) who are a little ways ahead of us in the journey of Eternity. Notice we hear the strains of this haunting tune in accompaniment to the film's closing shot. Death, redemption, and...love.
(BTW, I know you had a birthday anniversary recently, bbm_stitchbuffyfan. Happy birthday anniversary, belated though my greetings are!)