According to the History of Middle-earth (Volume 5, The Lost Road, 152 & Volume 11, The War of the Jewels, 348), Elrond and Elros were twins. The Quenta Silmarillion never says they weren't. But after the Ambarussas I don't feel like slashing yet another pair of twins. Therefore, this is an innocent story, except (disclaimer) that I stole all the characters from Tolkien.
Quote: '...Elrond Half-elven, who chose, as was granted to him, to be numbered among the Eldar; but Elros his brother chose to abide with Men.' (QS, last chapter)
'What are we? Do you know that?'
The old shores were destroyed in the War of Wrath; it was high above a new shore that two brothers, alike in face, sat gazing West. The sea was quiet, and so was the land, for these parts of the world had never trembled under the feet of the Valar, nor drunk the blood of the slain. They did not lament and they did not rejoice yet; they were waiting to be conquered by story, and song, and new sorrows.
'We are the offspring of elves and men,' answered the brother for whom the question was meant. 'Half elf, half man. There are no others like us.'
'Yet we cannot be both. Men die and leave the World. Elves remain in it as long as it lasts. We can not leave and stay, live and die at the same time.'
'So what would you choose?'
'You tell me first.'
They laughed, briefly. A seagull cried, high above them, while a sudden gush of wind pulled at their clothes and their long dark locks. They remained silent for a long time, but at last, one of the brothers spoke again.
'If you chose to be mortal, Elrond, what would be your reason?'
Elrond thought for a while before he answered: 'The Valar have said that after the end of the World, Men shall join in the Second Music of the Ainur. But no one but the One knows what will happen to the Elves. Perhaps they will die with Arda Marred, and be no more. If I would have the certainty of eternity, I would choose mortality.'
'Ah, yes,' said Elros, 'but the End of Arda is remote, and have we not discovered in our talks with Men that the nearer a thing is, the closer it is to their hearts and the more important to their minds? This would not be my reason. Mine would be freedom. The freedom to shape my life beyond the Music of the Ainur that is as fate to the Elves, or maybe even against it, to know that nothing is fated and nothing impossible, like our forefather Beren did.'
'If you go against the Music, you resemble Melkor,' Elrond said sternly. 'And yet, no theme can be played that has not its uttermost source in the One.'
It seemed as if Elros wanted to say something more, but in the end he did not; instead he asked: 'And if you would choose not to be mortal, what would be the reason? The fear of growing decrepit? Death ineluctable?'
Elrond shook his head. 'No, for I would have the gift of laying down life before death took me, and I would surely use it, because it is thankless to throw a gift away.' He laughed, but quickly sobered again. 'What I would fear most is to waste the one, short life span granted to me. To know, once my time was over, that I did not do what I could have done, and that it was too late to amend it.'
'I can see that,' Elros mused, 'yet one who does not waste his mortal years, will at least die thinking his life did make a difference, while an Elf runs the risk of learning that nothing he ever does is more than a passing ripple in a vast ocean. Mortals, too, can have their hopes wrecked and see their illusions founder, but the builder of a city seldom witnesses its fall like Turgon of Gondolin did.'
'So that would be your reason not to be counted among the Eldar?'
But before Elros could reply they heard an eerie sound, as if the wind made a hollow or a crevice in the cliff face reverberate with grief. They could almost imagine someone sang a lament on the shore, except that the words were lacking.
'What is that?' they asked simultaneously.
Neither of them had an answer, or if they had, each kept it to himself. They remained silent and listened for a while, until nothing was left of the voice but a sea breeze against the hard rock.
'It was your turn to answer, not mine,' Elros finally broke the silence. 'If you would not be counted among the Eldar, why would that be?'
'World-weariness, I think. Having seen it all, not once, but scores of times. Waking up one day realising there is nothing new under the sun, and never will be. Having had enough of the feast, yet not being able to leave because the hall has no doors.'
Elros frowned slightly. 'Indeed, it does not sound appealing. But I must confess that I feel too young to imagine such a weariness. What would deter me' - he paused for a moment - 'is the possibility of becoming like Maglor.' His voice held a note of abhorrence.
Elrond had been gazing down to where the surf met the pebbles of the beach; now his head shot up and turned towards his brother's face. 'Now what do you mean by that!' he said sharply.
'Nothing that will upset or anger you.' Elros briefly touched his brother's arm. 'I know that long as it took, he conquered you before he conquered me. Yet in the end he did.'
The grey eyes of the twins met, bright, piercing, unflinching, and neither looked away.
'Do you remember the night when he sang the Noldolantë to you? The night Father's star first sailed the heavens?'
'Of course I remember,' Elrond replied, 'but how do you know he sang it? You had left to do your star-gazing elsewhere, because you did not want to share it with him, and I never said anything about it, thinking you would not listen anyway.'
'But how could I possibly know, unless I stayed and heard him sing?' Elros countered.
'Why did you stay, then?'
'Because Father's star looked down on Maglor as well as on us, and made him lament the past. Something like that.'
'Father's star!' said Elrond. 'Maglor. You keep calling them so, though father is he who raises the children that are in his house*, teaches them what he knows, shares in their joys and pains and suffers their whims and follies. When did Eärendil ever do that?'
'We have been over this before,' Elros sighed. 'He had a mission to fulfill. Doubtlessly his heart bled when he had to leave us and Mother. Was our childish wailing more important than the unnumbered tears of two races?'
'I conceded you this point long ago. I only claim that he had a price to pay. The defeat of evil was bought with a parent's heart.' Elrond lay back in the soft grass. 'But shall we let it rest now? Tell me why you dread the possibility of becoming like Father.'
'Because if I became like Maglor, I could not die from my own wrongs. I could not die from my own bad memories. The past would never be dead; it would always be my own past, and detachment would be impossible. I would suffer without end.'
Elrond closed his eyes, and his face set. 'But you would have the time to change your heart. The time to face the truth again and again and again, until it became as familiar as the face of a beloved one who will forgive you all the wrongs you might confess. One lifetime is not enough to learn all the lessons one needs to learn.'
'That would be a reason for you to be counted among the Elder race?' asked Elros. He rose, and seeing this, Elrond sat up again.
'Yes. But you cannot go yet. You have not given me yours.'
'A reason to be Elvish?' Elros bit his lip in apprehension, hard enough to make it bleed. 'I cannot think of one,' was all he said.
Elrond scrambled to his feet, his eyes widening. 'So you have chosen...' he whispered. All the blood drained from his face.
The reaction of his brother mirrored his. 'Then so have you, or you would not...' His voice trailed off.
'But we are twins,' Elrond began, 'so how...'
'... can we choose so differently?' Elros finished the sentence.
They faced each other, each perceiving the panic of the other. Then, they were in each other's arms, embracing fiercely, clinging to one another as if to resist the forces that pulled them apart.
'If you become mortal, you will never see Eärendil - Father again,' Elrond murmured. 'Or Mother.'
'Perhaps...' Elros began.
Was there a flicker of hope? 'Perhaps?' urged Elrond.
'Perhaps all my arguments, and yours, were nothing more than air.'
'So we merely become what we are?'
Silence, once more.
'One day, you may see our parents,' Elros said finally. 'Give them my love.'
For the moment, there was nothing left to say. They let go of each other, stepping apart. And the wind rose again, and wailed among the cliffs.
*not just a remark, but one of the traditional ways of Elves to refer to their children (Laws and Customs of the Eldar, HoMe 10, Morgoth's ring, p. 228, Note 3. This notion may very well be crucial to the Elvish perception of parenthood.
Last remark: the first person to spot the reference to Nietzsche will get a review from me...