Suffered in similiar situations many times..
so heres some info:
if both cats are male and adult , you are in trouble ..
if both cats are female and adult: you can solve the problem within few weeks..
if they are different (male , female) : you will have babies after 
So for solving the problem (if you will keep the other)
you must love and give importance to the other in the eyes of your
old cat..Under any fight circumstance punish the cat who start the
fight..Feed them together but keep their plates seperate (min 1 meter apart)
within few days fights will diminish..
But if they are adult males during their seasons to find females they will pee everywhere(territory sign) and again fight for their territory..
So if somebody stays in home during day time its ok..Can keep an eye on them..Otherwise you need to keep them in different rooms or cages during these times..
I know what you mean Cem... I got one of my previous cats in this way although both adult Males they were OK...
This time I can't let it happen. They've been fighting constantly and TBH I don't want another cat!
if you decide to give the cat somewhere else its ok..
but if you want to stop the fights you need to punish(spray them water, take in the hand and give a knock in the head

) them immediately after fight..if you scare them enough this will solve very quickly..I behave the cats very softly so none of our cats listen me

But when wife says something they become like a soldier

ps : some home cats are really really capricous ..and stubborn..
we have one female named "princess" ..trust me we have suffered with this very long..Dont eat normal things..Only eat expensive dry food and meat..(from its rather previous owner)..
But my wife trained -them- very well ..Now -they- even eat spagetti..
