the elder one is just saying im the boss!
to fix this give the elder one more attention than the younger one when they're together.
but when the younger one is alone give her/him attention without the other one knowing.
So the elder one thinks she/he is the boss and now has no have need of to hurt the other one, only maybe by playing, and the younger one is getting the same attention and will not hurt the other or receive greedy.
Answers: Dont seperate them, it will just take that much longer for them to get used to respectively other. The one kitten is beating the other because he's been in your house longer NOT because he's elder. All cats do this, its territoral. (maybe put them in seperate rooms when no ones around to break it up) Eventually they will be playing & sleeping together. I know its scary, but its completely normal. The elder kitten WILL NOT kill the little one, he's just gonna beat him to show whos boss.