I don't know, in Companeros I actually think it was subtly suggested since the beginning, when Milian is with his men bragging about his new role as the general's lieutenant and you see the girl looking at him from a distance. Same goes for when Milian cuts her hair, he's taking it personally there, lol. The way I see it, they always liked each other, but Milian had to "grow up" in order for the romance to happen. In the beginning he's more like a kid who pulls the hair of the girl he likes.
I tend to see the relationship in the Great Silence more like a hurt/comfort thing rather than a "OMG TRUE LOVE!" one. It's the meeting of two outcasts who bond over their solitude and ease each other's pain (gosh I'm getting lyrical here). It's a budding romance but um, we know how it ends. Anyway I find it beautifully played, in spite of it happening in a short timespan.