I think that the arc of each characters' narrative is fairly fascinating in addressing this question. They start off diametrically opposed to one another. Then, they move closer to one another due to their captivity. Eventually, their love for one another causes them to share narratives. Yet, this convergence is temporary because once Molina is released, he lives the brief life of a revolutionary, something more akin to Valentin, who dies with a sentimental vision of happiness in his mind, something more akin to Molina. It seems that this arc reflects how each character fundamentally belongs to different cultures simultaneously.
The fact that each one believes that their cultural understanding of reality is a valid one would display how they do not sacrifice their belief systems. To a certain extent, I think that each one initially believes that their value system is inherently superior to the other's. Valentine spits nothing but venom at the stories that Molina clings to while in prison. For his part, Molina cannot understand how the revolution will do anything to enhance the sense of the aesthetic and the beautiful, to which he devotes his life. This reflects how each character does reflect a different sense of belonging. It is through their love where each is willing to accept the other's difference into a joined union. It is not a vision where one partner relinquishes their own notions of the good. Rather, it is a realm where differences cease in the arms of another and within the heart of another being. When Molina is released, his pursuit of a revolutionary end is not because he sincerely believes in it. Rather, it is because he believes in Valentin and the love they share. When Valentin seeks escape in a vision that is different than reality, he validates his own love for Molina when he embraces a vision that comes out of one of Molina's own creation. Love and the concept of a shared relationship is where the common ground is found, allowing each to live in their own world and culture while being immersed in that of the other.