Interesting question and query. I think that one main reason the story is so compelling and gripping is that there is no control by either man. you are right, chelseagirl, that many if not most relationships have a control dynamic to them, and usually they emerge wherein one person controls "their turf" and the other controls "their turf" as well; they split up the controlling and the responsibilities. Such tends to lend itself to reliances that keep people together, forge familly structures and order, and give individuals the self esteem they need/want.
Ennis and Jack on the other hand were more independent, interestingly transparent, and without pretense. Ennis in particular wouldn't know how to control if he had to. In spite of his working around Alma, I would not define that as controlling; he just used near childlike excuses and insensitivieites to go after that which he wanted.
the awe and beauty of Ennis' view of their relationship was essentially unmeasured, pure, and without reins, as he said. Any controlling was on Jacks' part, imo. Ennis was clearly nervous about having to tell jack that he would miss the August visit; Ennis knew Jack would have a fit, which he did. Hence, that Ennis knew Jack was a bit of a brat, Ennis' natural behavior was tempered, hence controlled by Jack somewhat. Had it not been, Ennis would have announced this August change of plans up front, but he held off due to his fear of Jack's reaction.