Warning: This one-shot floats on the verge of dreams and wakefulness... You should pay attention, if you don't want to get caught up in this complicated cobweb of nightmares and emotions! :)
Most importantly though, you should enjoy!

Sleep always came slowly to him. In this, they were different.

He sighed and sat up.
This routine of the late evenings was tiresome but he grew used to it, like he grew used to the scar on his left hand or the voice of birds greeting dawn and waking him from his hard-earned rest. The blood staining his cloak in every battle, the sight of his mother's empty chair by the long table in the hall, the absence of Estel, he grew used to these.

Acceptance, he learned early. Mute acceptance.

After pouring a glass of wine, he walked to the windows. He watched as the light-colored curtains swayed gently in the soft, warm breath of the summer night. He heard a quiet, troubled moan and looked at Elladan. A lock of dark hair rested across his brother's face, dancing, fluttering there like a harmless little wilwarin. Elladan swept it away with one swift motion of his left hand, without waking up.
Ticklish.

He smiled slowly to himself. In this, they were alike.
And he pulled his own hair back, sticking a pin into it, to make sure it would not irk him in his silent meditation. Perhaps he should cut his hair short after the fashion of the Rangers. It must be so much more comfortable. The elves would probably laugh at him. Elrond would be mortified. And he most likely would not care much, so long as Elladan joined him.

Perhaps one day.

He stared into the stars and frowned. Unlike the rest of his kind, gazing at the night sky was not refreshing or placating for him. Their sight invoked a torrent of different emotions; anger, sorrow, loss. Pain and questions, hundreds, thousands of them, roaring like wild storms of the mountains. And unanswered they left him, flying away on the air of a deep, frustrated breath.
He drank his wine and lay down once more.

Sleep never came easily and he was used to it. Instead of peace, troubles haunted him during the night, memories he was able to banish from his mind during the rush of the day, when he had Orcs to find, battles to win, innocents to protect.
Instead of trying to sleep and failing with it any further, he got up. He dressed quickly and taking his sword and bow and quiver, he left the room through the windows. Much like Estel used to, he smiled to himself.

Once he was outside, he ran. Uneasiness came over him that soon grew into mortal fear. He jumped over streams and clefts, ran through thick forests, letting the branches tore his weapons, his clothes, his skin, leaving bleeding wounds all over his body.
He did not even feel the scars, he ran with all the speed he could muster. Why and where he run, he knew not.
Suddenly, he arrived to a meadow. The night was dark, the stars disappeared and he halted.
A bolt of lightning lit the forest with hideous light and he saw a figure in the middle of the meadow.
He was tied to a huge tree, completely naked and tortured; skin and flesh exposed to the wind that grew stronger by the minute.
He hurried to the miserable creature, only to see that he was beyond help. His face was beaten and transformed beyond recognition, his body dark with the blood that oozed from his many wounds. His arms were cold, limp and lifeless. He reached for the artery on his neck without thinking, the teaching of his father rooted deep in his mind. With his last strength, the poor soul lifted his head now and placed his face into his palm. The chopped tresses of his short, dark hair pricked Elrohir's fingers with their coarse end.

Their gazes met, recognition shone in two pairs of identical grey eyes, and he gasped for air as the last breath of life left the tortured elf...
And he sat up with a deadly cry.

'Hush, brother!'
Elladan tried to get up to comfort him, but his hair got stuck in the complicated ornaments of the headboard.
'Valar damn this hair,' he cursed 'That is enough. I will cut it short, I sw...'
'No!'Elrohir cried, face ashen, terrified. 'Don't you ever dare to even think of it!'

Elladan finally managed to free himself and walked over to his bed. He poured water to his brother, who still stared into the darkness of the room with wide eyes and dried mouth.
'Bad dream?'
He nodded.
'Involving hair styles?' He asked, his voice filled with mirth.
'Do not jest with me, not now. Just promise you won't ever let anyone cut your hair!' And he drank the water.
'Don't be a fool, brother!' Elladan smiled at him. 'I can't promise such a thing. But I shall not cut it myself, if that puts your heart at ease.' He rose up and brushed Elrohir's head with light fingers, removing the pin.

'Losto vae!'

He waited patiently, listening to the even breathing of his brother, until Elladan sat up with a fearful cry.
Elrohir reached for the water and gave it to his brother.
'Bad dream?' He smiled.
'Don't ever cut your hair, brother!' Elladan nodded. They both felt the irony and for the sake of each other, they managed a shaky smile.
'I can promise you that, if it puts your heart at ease...' He lay back in his bed.
'Losto vae!' Elrohir added, needlessly. Elladan was already deep in sleep.

He rested his head on his pillow, sighing. Sleep always came slowly to him. In that, they were different.
In everything else, they were one, it seemed.

Losto vae, surprisingly means "Sleep well!".

For some reason I imagine the twins to be left-handed and filled with fears. These two facts somehow seem gravely (and equally) important to me. :)