I think Tommy would hide it for awhile. It's not unusual for him to wear long sleeves and multiple layers. He isn't trying to deceive Buck, although the guilt eats at him because that's exactly what he's doing, it's just--he tells himself it doesn't matter anyway. What would Buck want with a soulmate like him anyhow. And he means to keep his distance, but it doesn't work out that way, though I keep I would hold off on their first kiss much later than in canon.

Maybe Buck figures it out before Tommy tells him. He didn't think much of it at first when he would catch Tommy looking at the scars on his legs because they're visible for anyone to see. He doesn't question the way Tommy's gaze lingers on his chest during a sparring session because it's not the first time he's caught Tommy's eyes lingering on him in an appreciate manner. It's the little scars that give it away. Buck can't see it anymore, but he had a scar when he was younger along the knuckles on his right hand from when he fell out of a tree when he was 9. He and Tommy are getting coffee and when Buck slides Tommy's coffee across the outdoor table, Tommy's eyes linger on Buck's hand. The first time Tommy kisses him, Tommy's lips connect with the right corner of Buck's mouth and even though it isn't noticeable now, when Buck was 11 he crashed his bike into a tree and bit a hole right through his lip.

For some reason, Buck doesn't tell Tommy right away that he knows. There's a lot of insecurities from both parties and so many things that go unsaid, but it gets to a point where they are all but admitting it to each other. When they're lying naked in bed and Tommy's tracing the path of Buck's Lichtenberg scars telling him how beautiful he is. When Buck lets his lips linger on the inside of Tommy's left thigh right over the healed scar from a fishing lure Tommy got one summer on the lake when he was 13.

The way Buck brings it up is by asking Tommy when he's going to let him see his scars, though they both know he already has. They both know he's really saying when are you going to let me in?

Maybe Tommy deflects, smiles at Buck and says you first, sweetheart.

I don't know exactly where this would go or how and when they finally stop being so scared of the enormity of being each other's soulmate, but also realizing that doesn't mean they wouldn't have chosen each other anyway, that they have been choosing each other every day pretty much since they met.

Before they can do that though, maybe they break up. Maybe it's a mutual decision, maybe they want to prove to themselves and each other it's real and not just because the universe or whatever says so. Maybe it's less mutual, one leaves but the other doesn't ask them to stay. When they inevitably run into each other months later after no contact and they hook-up...right after they've helped each other out of their shirts, they both notice a golden jagged line right over the other's heart.

Tommy traces the new line on Buck's chest. "Oh."

Buck does the same. "That's new."

They barely make it to Tommy's bedroom, pressing each other against every wall, barely letting each other breathe.

After, Buck's tracing the line on Tommy's chest again. "Does it hurt?"

Tommy kisses Buck's forehead. "Not anymore."