You need to re-run the introduction process. What behaviourists recommend is
putting one cat in a room on her own with the other outside so that they can
smell her but not reach her. The next stage is to bring the new cat out in a
cat carrier and allow the other cats to look. Then reverse it, put the other
cats in a room and allow the new cat to have a good smell and explore. A key
product for this is the Feliway Diffuser. Links below (note the article on
Pheremonatherapy as well - this is Feliway).

Multi-cat households can be fraught with difficulty. Unlike dogs, who are co-
operative hunters, cats hunt alone and have not built-up a social repertoire.
At the end of the day, you may have to live with an uneasy truce. The UK
behaviourist Vicky Halls writes eloquently on this. If the cats cannot bond
(i.e. you see no mutual grooming or facerubs), then the best you can do is
reduce conflict over access to favoured areas. The idea here is that, if cats
realise that what they covet isn't in short supply, they don't need to fight.

Littertrays - however well the cats get on, one litter tray per cat, plus one
to spare, and not all together, in different areas of the house.
Individual feeding areas and water stations for each cat.
Plenty of beds.
Access to high resting places - plenty of cat centres.
Plenty of scratching posts.
Plenty of secret hidey-holes.
Toys galore.

This is Vicki Halls' list of covert tactics to watch for:

Dominant cats blocking stairways, narrow corridors, doors and catflaps.
Jumping on the victim while she is asleep.
Intimidating the victim by staring, poking or blocking while she uses the
littertray.
Staring to remove a victim from a place of comfort.
Stalking.

The basic battle is a territorial one. Safe access to the outdoors would help
a lot. Can you catfence your garden or yard?

http://wizz-catz.co.uk/introducing.html
http://www.fabcats.org/behaviour.html
http://www.sniksnak.com/cathealth/aggression.html

f.ck the behaviorists. The cats will work it out among themselves. Be
patient.