I own nothing but the plot
It had started out as supporting each other, the only comfort they could find between the cold cells or being experimented on they didn't have much else. Yet it developed into more as they sat backs to each other and hands joined with only a few steel bars separating them. If either of them had the strength they would've broke them, then again if they were that strong they wouldn't have been stuck to start with.
Time worked differently there. You couldn't count seconds or minutes it would drive you insane, but you could take notice when one of them was taken for the next round of experimenting which would mark one day. Eventually they stopped counting the only reason they did it was to plan revenge and make the sick bastards doing this to them pay for however long they were there. Yet as the days turned to night and the torture continued the idea of escaping became a far fetched dream and even planning revenge became tiresome.
It was when he knew his companion was losing the very hope keeping him sane he revealed his plan, one of them drank both rations of blood for a year and then brake them both out while they were being shown off like animals. Neither of them was optimistic considering all that could wrong with the plan but they agreed to try if only to hold onto their dreams of a future together. They spoke at night in hushed tones of king sized beds and all the pretty little necks they could drink from if, no when, they escaped. They shared stories of what they would do to each other when they could hold more than just hands, when they could live out their revenge plan (they had finally settled on one) together without anyone to stop them.
They bonded more in that time with the tiny spark of hope building between them than either of them had with anyone while they were free to roam around outside. That's what it had become to them now, outside the place they could be free and the closer to the New Year it came the more it stopped being some fantasy and became a possibility.
The torture didn't stop, obviously. Hope didn't quite have enough power to stop such things but it stopped the small bouts of depression after bad sessions they were both prone to and it allowed them to share more with each other letting the excitement build and fester between them. Yet along with the hope and the excitement and the longing both vampires were letting consume them there was the doubt. Doubt about would they really stay together would their plan work and more importantly what would happen if it didn't. They both tried to push the thoughts away remaining positive, but they both felt it and it showed in the way they pressed harder together even with the bars still between them and the way every night their hands crushed each other tighter.
Finally it was time they had it all worker out how it was going to go down, and what they were going to do afterwards, killing and feeding from the fools who were so desperate to see the amazing pet vampires, then they were going to find the biggest nicest hotel in the immediate area and they were going to fuck and feed till they had to stop. Then they would hold each other like they had wanted to do for so many nights and then they would repeat the routine until they got bored and finally wanted to move on. The plan suited them both perfectly; the only thing they weren't planning on was ultimately their undoing.
Neither was ignorant to the effects vervain had on them yet they were surprised the doctor was that thoughtful so as to coat the cage in it, alas he did and neither of them could prevent the events that soon followed. The cage not opening was the second bad thing to happen the first being the fire consuming the room where the vampires were currently stuck. Well were one was currently stuck. The other was free to go, to live whatever life he wanted and the only sacrifice was the one person he had ever felt a strong connection with, the one person he wanted to keep with him for the eternity they both had.
In the end survival won out, he took a last look at the person he was pretty sure he loved and he flicked the switch which allowed him to turn his back on the screaming figure of his, well he didn't know what he was. Something died inside of both men that day and it would never be repaired.
Later on he was asked outside a damp bar what could possibly be so bad it made the great Damon Salvatore flip his switch, he didn't remember what he said but he knew it wasn't true. No one could know it was the fact he had betrayed the one person he knew wouldn't have done it back.