In the beginning, they were forced to be cautious. They were almost never seen together in public unless they were at a dinner party or a business function that involved both of their fathers, in which case it didn't seem odd that they made small talk with one another and with the other guests. They were interacting as rivals, not as friends and certainly not as lovers.
They got around all the secrecy though. It was a well-known fact that the members of Ouran Academy's infamous Host Club continued to be close friends as they grew up and entered the adult world. They met up for lunch or dinner regularly, and often took turns going to each other's homes and spending a few hours relaxing and having fun as they had in high school. Some thought it strange, but for the most part no one commented on it. Tamaki Suou was an odd boy, one with the strange ability to make everyone around him forget their cares and live only in the moment.
Their meetings weren't originally contrived to give Kyouya and Tamaki a chance to be with one another romantically, but they somehow evolved to serve that purpose in the end. It was only when the two of them were with their friends that they could be themselves. Hani was happy for them, which meant Mori was as well. Hikaru and Kaoru didn't mind, mostly because it gave them something to make mischief about. And Haruhi certainly didn't care if Tamaki pressed a playful kiss to Kyouya's temple when he thought no one was looking, because she was the one that had calmly informed them they were in love back when they were third-years.
Eventually, the two of them started disappearing into an unused bedroom after the social niceties had been observed. It was then that the lingering began. The first time, they lingered in the Hitachiin's guest room because Tamaki was horrendously embarrassed by the thought of facing the others immediately after they had finished making love. So they relaxed in one another's arms and chatted idly for the better part of an hour.
When they did rejoin the others, Hikaru and Kaoru were the first ones to speak. "You two were gone an awfully long time—is Kyouya-senpai really that good in bed, Boss?"
After a while, they began lingering just behind the front door. Kyouya was often the first to leave, and when he would reluctantly stand up and gather his things, Tamaki would just as reluctantly follow him.
"Good night, Kyouya," Tamaki would say in a small, sad voice that broke his heart every time.
He would reach out for Tamaki's hand and lean down to kiss him tenderly. "Good night, Tamaki."
Sometimes, Kyouya would wipe rogue tears from Tamaki's face before whispering one final good night. Other times, he was the one with damp cheeks and Tamaki the one doing the consoling.
It was different now that they had both been abandoned by their families. They could be together however they wanted to be. They could go on dates, share an apartment, and generally just go about their lives as any normal couple did. Neither of them needed exorbitant wealth or their families' legacies to be content with their lives. They only ever needed each other.
And if they still lingered on occasion—well, old habits were rather hard to break.