They were not dissimilar. Vexen was cold and Saïx was distant, and it wasn't that Vexen was any warmer to Saïx or that Saïx was any more friendly to Vexen; it was rather that they did not expect anything else of one another. So it went.
They were not similar. Vexen had been a scientist in his first life, a biologist, an examiner of minutiae, a rule-maker and rule-tester, a cataloger. Saïx had been a mystic in his first life, a soothsayer and magic-maker, a student of the holistic, a symbol-follower. So they had nothing to discuss and everything to discuss. So it went.
Half the reason was because they were bored. Half the reason was because they were curious. There was a third part to the reason, but it was one so small that it didn't deserve an actual ratio.
(The third part to the reason was this: they both wanted to confirm something they couldn't risk revealing to anyone else.)
It began before the Organization was split, in the castle that was their first second-home (Vexen's true first home was another castle entirely, and Saïx's was a hill, a grove, a sacred mound). It began when they were eight but not yet nine, when none of them yet knew why their Organization was numbered Thirteen, and it began almost by accident.
The boredom began with Vexen, and the reason for it was this: hearts had limited scope for his research talents. He didn't want to admit it. He certainly couldn't have confessed it to Xemnas. But he had analyzed every specimin of Heartless they had found, he had taken apart and put back together the dark matter that formed their bodiless bodies, he had studied the way they consumed hearts . . . . there wasn't much left. In some ways it was true to say that he'd hit a wall. In other ways it was more true to say simply that he'd lost interest. One subject wasn't enough to be scope for his talents. In his first life, he had had the entire scope of life to study.
The curiosity began with Saïx, who knew them least well (except for Axel; but that hardly counted, since Axel made a great show of not especially caring to know them well). The curiosity began with Saïx, who was the first of the Nobodies who had not known each other when they had hearts, who was the first to not have a legion of memories of what-it-was-like-before, about the others.
And they spoke of innocuous things: Vexen's research and his frustration with it, Xemnas' plan, Kingdom Hearts, until Saïx's hand landed heavy on Vexen's shoulder. Vexen looked at it, and then at Saïx, his eyes heavy with abstract curiosity, and he said, "What exactly are you doing?" But mildly, without accusation.
"Trying something," Saïx said, and then he leaned forward—without haste; slowly enough that Vexen could have backed away, had he wanted to, and slowly enough that he was able to process that thought and decide that he did not, in fact, want to, that he was as curious as Saïx.
So it was Vexen who moved to complete the motion, to bring mouths together, and when they touched, he thought, 'ah.' His hand on Saïx's chest, against the zipper, was . . . .
And it went from there onward, the clash of Vexen's rigor and Saïx's intuition, slow and deliberate and silent, sliding beneath robes and the brush of long hair, and all distant as the snow moon.
On the floor, white floor glowing, as white walls glowed—all bright in this place, bright and silent and hollow—Saïx said: "I want to know—"
And Vexen said, "Hush," because to discuss the desired results would contaminate the experiment, and because if Saïx was searching for something different he did not want to know it, and did not want him to know what he knew—
It went awkwardly at first, and neither of them was willing to admit that they weren't quite sure how best to make it better—until Saïx slid down and Vexen shifted and then it worked. And they were both, in their own way, able to take a cue and run with it.
Afterward it was not awkward, because they both knew what this was. Vexen righted his robes and Saïx smoothed his hair, and when Vexen reached out to push a whiteblue strand back behind Saïx's ear, from where sweat had stuck it to his face, well, that didn't mean anything particularly either. It just was.
But Vexen's suspicion was confirmed, and he thought that he might, perhaps, seek this out again—for further confirmation.
So it went.