when i took on the 2nd of mine she was very aggressive to my first bullied him, guarded trays and food etc, chased him about. she never went for me though. i thought the situation was impossible and as i had only taken her in to find her owner (she was stray but wearing and old too tight collar when she was found) resolved to rehome her. before i could, christmas arrived and they both went to the cattery for a few days, seperate pens but next to each other. when i picked them up she was better but still obviously intimidated my first but over time (2 years!) they have gotton to know each other and now play together, defend the garden together and do occasionaly sleep together. i think the change started when she realised this was home, that he wasnt going anywhere whatever she did and that there was enough food for them both, enough soft warm places to sleep etc.
it depends on personality and how determined they are to get their way but i wouldnt allow the new cat to guard and if i saw her, removed her to allow my first cat to go where shed been stopping him. and when he saw me step over her when she was sleeping on the stairs he started jumping over her.
i think some cases no solution can be reached but some of the books ive read suggest some things-
you could try distractng him with a toy when he looks like hes about to do something you dont want (guard/chase/attack)
i would be segregating the aggressor if you feel you need to segregate one- at the moment hes winning as the cat he doesnt like has gone away. this might not be possible if the only room available is your room and you feel threatened.
his aggression to you might be based on the stress hes feeling trying to remove your other cat. or if you are feeling unhappy with his behaviour your body language might have subtely changed to something less welcoming.
alot f cat books emphasise inteactive playing to relieve stress, promote positive feeling, distract from battles
Some books suggest restarting introductions from scratch, going at a slower pace and paying more attention to positive experiences in the others prescence (nice food, play, affection)