It's a bit like "I can believe two impossible things before breakfast" - but I think Ennis really sees the two thiongs as entirely different. In a way Alma is real life while Jack is the dream.
But what a limited real life it is - and when Alma suggests going out he has some objection - makes sense that he doesn't want to go out with her and Jack (!) and to me it also makes sense that he doesn't want to go to the church thing but it gives the impression that for Ennis life with Alma is just work, eat, children, sleep, repetitive dull routine, whereas for the short periods he's with Jack it's all exciting, full of feeling,like a holiday, as one wants life to be but knows it can't be.
He never seems to think that Alma may find her life tedious or unrewarding, lacking in material comfort, lacking in emotional warmth, lacking in social contact - when you think about it, Jack is right in a way when he says "That's a life?"