I, too, think it would have worked. For all the reasons others have mentioned before: similar background and values, they shared many things and completed each other in other aspects. They had a great love for each other that lasted through 20 difficult years. I don't think that living together would have been more difficult than living apart.
And, additionally, I'm a romantic and believe that there are people who are meant to be with and for each other. THE one person in the world who is meant for you and vice versa, no matter what. And I think this is what Ennis and Jack were for each other.
But when? When they were 19, not parting after Brokeback? Maybe. But both were so young and unexperienced then. They were both still teenagers.
In general, I think that you have to be older than that to be mature enough for lifelong commitments.
On the other side, my husband and I were 17, going on 18 when we came together. And it works for more than 20 years now. So it is possible. But we had not to fight homophobia (internalized and from other people) and poverty with little prospects. ("Pair of deuces, going nowhere").
After the reunion? Yes, I think it could have worked then. But I can't see Ennis leaving his family at this time, the girls still so little. Just for the sake of discussion: even if he hadn't been in love with another man, but another woman (ot there was no homophobia at all in the world) and all his fears and demons wouldn't have had a voice, he still would have stuck with what he got. Out of loyalty and duteousness.
After the divorce? Yes. I guess the post divorce scene is the moment in the movie when we all want to yell at Ennis: go for it, don't let Jack down again. This is your chance!
Later, at any time between divorce and lake scene? Yes.
At the end, around or after the lake scene? Again: yes. We've discussed the "What if Jack hadn't died?" many times and I'm still convinced that Ennis was almost there, almost had made all the way around the coffeepot, had almost found the handle (paraphrasing Lee here, because her analogy is so beautiful, hope you don't mind. BTW: Amanda (?) once said something about 'creating our own Brokieisms' and this phrasing is definately one. I've seen it couple of times).