I think that there are a couple of elements at play here. The most obvious element is the sexual orientation frame of reference. Molina is more of a "woman," reflected in his using first names. Valentin is more of a "man," in how he uses last names to refer to people. In this, one can see the most banal use of the sexual orientation stereotype. Yet, I think that the rationale might reflect more about their character, outside of sexual identity.
I tend to think that the reference of name is reflective of the fundamental difference in reality that each character holds. Molina is someone driven by the arts, as represented in his love of films. Valentin is driven by historical and political reality, signified by his Marxist revolutionary zeal. The difference in how each refers to the other might not be connected with sexual identity as much as how each sees the world. For example, Valentin is going to refer individuals by their last name because those connected with a socio- economic frame of reference understand reality in this light. Valentin sees the first name as meaningless, and perhaps would see names, themselves, as secondary to ideas. In this, he uses last names. Molina being a window dresser and one who loves dramatic reinterpretation of reality uses first names to address beings because he believes in the uniqueness of the names himself. Consider that Molina's original last name was "Molino," something where the changing of the last letter indicates a change in gender. This is reflective of how Molina does not necessarily view last names as indicative of anything, closely seen to Valentin's belief that ideas are more transcendent than names. In using the first name to refer to people, Molina represents the idea that individuals are characters in a drama, representing aesthetic notions of the good and can be referred to by their first name in reflection of it. The fact that Molina dies almost nameless in the street after being gunned down by revolutionaries adds to the intensity of how Molina's vision of reality might not only be preferable, but might be more accurate than anything else. In this, one sees how the use of names can be something constructed outside of sexual identity, and more reflective of their perception of reality.
Molina address Valentin by his first name because he wants to be closer to Valentin. The opposite is true of Valentin. Valentin is trying to distance himself from Molina by calling him by his last name, much as men who are on a sports team do. Valentin does not want to have a deeper relationship with Molina, but eventually, as you know, he does. He is drawn into it, despite himself, but the names remain from force of habit, and probably to keep the readers focused and to prevent confusion.