The name "Ennis del Mar" means Island in the Sea. And Alma can mean young woman, or even water, or something which flows, or it can mean Soul/spirit.
So, Ennis is separated from Jack by a gulf, a chasm. In the beginning of the movie and the story, it speaks of Jack and Ennis catching glimpses of one another from a distance, across that chasm of valley which separates them. "Twist" supposedly means the power of a rodeo cowboys legs to stay on the bucking horse, and ultimately break or tame him.
Jack only really catchs and tames Ennis posthumously, from beyond the grave.
This is all wild speculation on my part.
So, if Alma is the spirit and water of acceptible heterosexuality, and Ennis is an island, isolated in the middle of that water, but brokeback represents the process of taming, possessing, and there is an eternal dilemma which tears Ennis between his love for Jack, and his desire to conform.
Just some thoughts...