Only a damaged heart can think of one person to the exclusion of all others.
Subaru denies being a damaged heart. He thinks of people other than Seishirou. Just because he doesn't think of them as often, or as strongly, doesn't mean he's excluded them entirely.
Subaru knows he's lying, but can't help himself.
Seishirou denies having a damaged heart. Before Subaru, he thought of no-one at all, so really, he's healing if he's anything. Which he's not. Because he doesn't care for Subaru, either.
Seishirou knows he's lying, he just doesn't care.
They think of nothing but each other.