Handling it gradually, as you are doing, is the key. Once you move in for
good, continue to do a gradual introduction, separating at the first signs
of agression. Switch who is out of the room and who is in the rest of the
house regularly. It may take a while but neither should have to give up
their kitty.

[snipping to add a few comments]

[snip]

Good idea except I wouldn't do it on a daily basis.  I've
had friends who have had to keep cats permanently
separated, and they usually let one cat have the run of the
house for a few days to a week while the other(s) remain
confined to one room, then switch for the next few days
or a week.

I've had cats since the '60s and have moved a lot between
spacious houses and one & two bedroom apartments.  To make
it more complicated, my first cats were indoors only for the
first 7 years of their lives (apartment dwellers), and then became
indoor-outdoor cats for the next 11 years.

They've all adapted, even the older cats, even the most
high-strung Siamese.   :)  I think as long as you give them
fresh food & water, clean litter, toys, & visit daily, (& maybe
a radio or small TV for company), a cat can handle
confinement to a room for quite awhile.  They are sensitive
to you, so if you act like they're in prison, they might be
more upset then if you act like being in the room is a treat.

I think you're wise to take this into consideration.

On the one hand I'd personally rather keep the more mellow
cat, but cats do tend to mellow with age (even your elderly
Siamese might yet mellow - some!).  As a last option, if the
alternating confinement doesn't work, I would agree that it
would make sense to give away to a good home the cat that
is easiest to adopt out.

to make this transition as easy as possible!  Thanks again!

Good luck!

Gail
Happily owned by Lao Ma, Ephiny, Minya, Mattie & Harry

love

Good luck, I hope it works out.

some good advice from pam johnson-bennets Think like a cat forum

Cat to cat aggression
http://messageboards.ivillage.com/iv-ptthinkcat/?msg=14085.1

Introducing new cat
http://messageboards.ivillage.com/iv-ptthinkcat/?msg=13954.1

Interactive play time
http://messageboards.ivillage.com/iv-ptthinkcat/?msg=11667.1

I'd sooner give up my wife... in fact I did.   Her last words:  "The cats or
me"... She lost.

Just follow these instructions:

http://www.maxshouse.com/introducing_cats.htm

I would start exchanging their bedding and other items that have their
smells on it. So, each cat, in the current  home, starts getting the
other scent on a regular basis.

Also, when they are actually introduced, try rubbing the blankets on
them so that they both have more of a combined smell. if they smell
more alike, they probably won't be quite as agressive. We used to use
perfume on both cats when introducing them. They were more confused
than upset.

When you take your own cat over, vary who gets which room while they
are spearated. They need to get used to each other being in the same
spaces, even if they aren't there at the same time.

You could also put the two on leashes and introduce them that way. You
control how close they get together, so any fights can be stopped. Buy
they can get close enough to sniff.

Several years ago, my mom's cat had a litter of kittens. My cat is
afraid of kittens, so he took one look, squealed, and ran away. The
mother cat went into attack mode. After that, anytime they saw each
other, Maynard would run in a panic, and Tov would attack. This
continued for months, long after the kittens were gone. It got so bad
that Maynard would hide behind the toilet, or up on my bookshelf.

This could not continue. He was way too stressed. Neither one of us
would dream of giving up our cat, so I came up with a plan.
Desensitization.  I live in the basement. So, I started by keeping my
door shut, except when I needed to go through. I had a sign on my door
as a reminder to everybody in the house (I also explained what I was
doing). My dog, and my 2 other cats were allowed to go up and down.
But Maynard was not to be let up for anything, and Tov was not allowed
down.

Because of the months of attacks, it took a few months for Maynard to
recover and be secure. I had to re-establish my room as a safe place.
He would actually panic if carried partway up the stairway. After a
few months, he would go up to the top on his own.

Once he was comfortable and relaxed in our room, then I started the
next stage of the plan. I would lock Tov in my mom's bedroom, and then
let Maynard up for a few hours. The first month or so, he was very
scared. He would hide and try to go back downstairs. The idea was to
get hom used to be upstairs without being attacked. He had to learn
that it was okay to be up there, and that attack was not imminent.
They also got used to smelling each other's scent in the common areas
of the house.

Once he was totally comfortable in the rest of the house, I started
bringing him up and holding him, while Tov was loose. This stage took
a couple months, and it was very important. I had two cycles to break.
Maynard would run because Tov attacked, but Tov attacked because
Maynard ran (he acted like prey).  

I could feel him tense up when he saw her, and she would tense and get
that predator look. They both had to learn to stop their previous
behavior. Maynard had to learn that Tov wasn't going to chase her, and
Tov had to learn that Maynard wasn't going to run.

Once he was relaxed when held, then I progressed to a leash. He had
some movement, but I could prevent him from running.

When that was good, then I switched it. I put Tov on the leash and let
Maynard loose. This stage actually went much better than expected. Tov
decided that she could not walk with a halter on, so she just laid
there on the floor, unable to go anywhere. She was actually quite
pathetic, so we started putting her in the halter and letting Maynard
loose.

We continued to supervise them for a couple more months before leaving
them alone with confidence. Tov and Maynard got along fine after that.
They were never buddies, but they could walk casually past each other.

It took almost a year for this process, but we let the attacking
problem go on for 8 months before starting that process. We had a lot
of damage to undo. Since you don't have the attack history, it should
be a shorter process.