yes separation is the answer,or have one be outside/inside Sounds like its about territory , I tried to adopt a male cat last year and mine a female was so annoyed it just didn't work..I got a good home for him ..I dont think fixing/spaying makes any difference to the territory thing ..hey hope you get it worked out..separating them does seem to be the answer.good luck
Try separating them into separate rooms..And put a baby gate up in one of the rooms and let the other cat come by and sniff but the baby gate will stop it from attacking the other cat..then lock the aggressive kitty up and let the non aggressive kitty out for awhile and let him mark things..the idea is to get each other used to one another's scent