I have always believed that Elladan and Elrohir chose the same fate, but sometimes it's interesting to explore other ideas...
Ch.1: A life for a life
The battle was hard won. Elrohir should have felt immense relief, a giddy sense of exhilaration and adrenaline that made one feel like they owned the world. That feeling was hard to conjure up as Elrohir knelt beside Elladan's quickly slackening body, staring in horror at messy wound where a broad sword had been forced all the way though the older twin's chest. In a daze, Elrohir ripped off his cloak and pressed the already damp fabric against the wound in an futile attempt to stem the flow of blood. Elladan's body arched weakly under the pressure, hands scrabbling in the mud for a purchase against the pain. The pounding rain was not helping. It washed the blood away as soon as it welled up, collecting in a diluted puddle under the fallen Peredhel. A few moments later Elladan moaned weakly as his eyes started loosing focus. Elrohir could see that he was loosing his brother.
"No, no, no, Elladan! Stay with me, muindor! Stay with me!" Elrohir's voice cracked as he gently tapped Elladan's face. The motion only caused Elladan to spasm in pain. There was moisture on Elrohir's face, but it was hard to tell if it was rain or tears. As though sensing his twin's distress, Elladan gathered what little strength remained and caught his brother's eye. Elrohir's crouched over Elladan to provide what little cover he could, the makeshift bandage all but forgotten as Elladan struggled to speak. It took several attempts before anything remotely coherent passed Elladan's lips.
"Ro...S'kay," Elladan gurgled as he choked on his own blood, each breath harder than the last, "L...Lo..L-love-"
Elladan didn't finish before his body shuddered softly as his fëa departed. With a wail of desperation and grief, Elrohir recklessly threw his own fëa after his brother's, determined to bring his brother back or go with him. He didn't get far before he felt a restraining hand on his spirit. Turning, Elrohir saw a ghostly figure shrouded in grey towering behind him.
"You can not follow him Little One. I would not take you both from you father," the speaker's deep, rolling voice left no doubt in Elrohir's mind. He was speaking with Lord Námo.
"No, please. If we can't go together, take me instead. He is courting; I have no one, I won't be missed. Take me instead," Elrohir begged, thrashing again the Vala's hold. He looked around for Elladan, but his brother's fëa had melted into the grey light, disappearing from view. Here, caught between worlds, time slowed to a crawl. Elrohir watched with detached fascination as the rain almost seemed to hover in the air around them.
"That is not the way that works Little One. You are not the first set of twins I have had to separate." Námo drew Elrohir's spirit back towards his body. Elrohir fought desperately to go the other way and follow his twin, but he couldn't escape the Vala's hold. Soon Elrohir found that he was looking at his corporeal body, collapsed on top of Elladan's. Elrohir renewed his wild thrashing.
"No! I won't go back! Don't make me!" he screamed, half sobbing.
"Elrohir, Child, must you make this difficult? Your father has already lost one son. Don't make him loose you as well to your grief."
"No, please! You have to let me see him one last time. He never made his Choice!" Elrohir protested as though that piece of information could sway the Vala.
"I know. His fate shall be the same as all Peredhil who didn't make a choice before their death: He shall be counted among the Second Born," Námo's voice was gentle, full of almost paternal sympathy.
"Is there nothing I can do to see him again?" Grief had driven away all rational thoughts. If Elrohir had been in his own body, he would have been hyperventilating.
"Not that I would willingly let you do."
"But there is something I can do?"
"Yes," Námo sighed reluctantly.
"Tell me." Elrohir frantically seized onto the thin ray hope like a lifeline. Námo muttered something along the lines of 'Manwë will be thrilled', but he gave into Elrohir's begging.
"Your father's twin never chose to be mortal. In fact, from my understanding, both he and your father planed on choosing the fate of the First Born. However before they had pronounced their Choice, Elrond was mortally wounded in the last battle in the War of Wrath," Námo sighed before continuing, "Elros was distraught to say the least. He knew that Elrond loved the live of the Elda beyond aught else. In an agreement with Manwë, Irmo and I, Elros chose to forfeit his Choice in order to save your father's life. In the trade, though, Elrond lost his Choice as well. When he fëa was returned it was that of the Elda, not the Peredhil. Elros gave up immortality to save his brother's life, but it did not save their relationship; their fates are forever sundered."
"I had no idea," Elrohir was at a loss for words. He had never heard this story. He had always assumed that Elros had chosen a mortal life, not that he had sacrificed his immortal one.
"Elros never told his brother the full story. Elrond, though, made an educated guess as to what had happened. He was mad at Elros for several decades. It wasn't until Elros lay on his deathbed that they reconciled."
"If it was done once before, then I will do it again. I wish to follow Elros' example," Elrohir declared boldly, though his voice catching in his throat a bit, "I wish trade my Choice for my brother's life."
"Elrohir, are you sure you want to do this? Think of your family and your brother, Child. Is this what they would want? Would you bring your brother back, but take away his Choice? Your fates will be sundered. Once it is done, there is no going back," Námo's gaze was sad and a bit perplexed.
"I'm sure," Elrohir said softly looking down at his brother's lifeless body.
"Very well. If that is your Choice," Námo looked at Elrohir for confirmation, "I will do my best, but this will hurt."
At Elrohir's nod, Námo gently reached over to seize his fëa. For a moment, nothing happened. Then the air shifted as a part of his fëa snapped free. Elrohir couldn't muffle a scream of pain as part of his soul was irrevocably torn from him. What was left of his spirit burned and throbbed. Námo murmured something Elrohir didn't hear. The pain subsided, but did not leave completely.
Then, all of a sudden, Elrohir was kneeling by his brother once more in the driving rain. Scrambling at his brother's neck, Elrohir almost cried in relief when he felt a faint, but steady thrum against his finger tips.