No idea if it would work, but if Pi went away for a few days, do you think
it might change the dynamic?

I know I'd want to at least get Pi in for a check-up to see if there is a
physical problem.

Jo

I wish I had something really useful to suggest, but I haven't
experienced anything like that. Though one cat may be suspicious of
another who has been at the vet's, they usually settle back in a couple
of days, at the latest. Have you tried Feliway or Rescue Remedy or
anything like that? What a tough situation! Purrs that you can resolve it.

You need to give this time. What you are experiencing is redirected
aggression and this is something you need to keep in mind and learn to
manage for any future vet visits because it will likely happen again.
It is a very common occurrence, mostly revolving around one cat
smelling different, but can be resolved with time and patience.The
good news is there is no need to rehome either of the cats. You can
work this out.

The first thing you should do is switch which cat is in the office
regularly so each one has time to run around the house. No need to
isolate the aggressor to the office all the time and you may very be
seeing some of this recent aggression because he is frustrated. Your
other cat can spend time in the office for awhile. It won't hurt him.

The second thing you need to do is replace your office door with a
cheap screen door so they can see each other, but not fight.

The third thing is to start feeding them really good canned food on
either side of the door so they will associate something positive with
the presence of the other. Once they seem calm you can try allowing
them out together for very short periods of time and gradually
increase the time together. Never allow them to fight and if you sense
something is about to happen just separate them and try again the next
day. Whatever you do, do not reprimand or otherwise punish either cat
if they react badly. It will only serve to create resentment and make
the situation worse. Even though it sounds kind of backwards, even
hissing or growling should be responded to with gentle, soothing tones
and the theme of creating positive associations with each other should
be maintained always.

It would also help to purchase a Feliway diffuser and keep it plugged
in close to where the office is. This is not a magic bullet, but most
times it is very helpful. If just the behavior modification doesn't
work, you still have hope. Sometimes a short course of medication is
used in these instances to calm the aggressor until the cats are
interacting fine for a period of time, and then meds are slowly
tapered, then discontinued. Good luck. It's NOT hopeless by any
stretch of the imagination. :-)

Thanks to everybody, for responding and for your support.  

I was unable to find a screen door that would work, but I asked on
freecycle and received a cage large enough for a cat to stay in it for a
few hours each day.  

The lovely lady who gave me the cage has rescued and fostered many cats,
and when I explained the problem, she said that I have a dominant cat
(Pi) and a submissive one (Doc) and that all along I have inadvertently
been sending Pi the message that he's Top Cat.

When I come home, I always offer my hands to the cats to smell, so they
can tell where I've been, and since Pi always shoulders Doc out of the
way, he gets to smell first.  He is greeted first (same reason) and since
they're fed together, he can shoulder Doc out of the way to eat first, as
well.  I suspect one reason that Doc is so thin is that Pi clobbers him
while he eats.  :-(

Whenever I offer them treats, I always offer to Pi first, because he
usually doesn't want what I have (he's a very picky eater) and then I can
give Doc the treat without getting Pi's nose out of joint.  
Unfortunately, that has sent Pi the message that he's the King around
here.  

Anyway, I'm now in the process of reversing that.  I feed them
separately, they each have their own water and food dishes.  I put one of
them in the cage (usually Pi, but sometimes Doc) and feed them within a
foot of each other, always feeding Doc first.  I give them food and
treats and catnip *only* if one of them is in the cage, to get them to
consider the cage as a pleasant place.  And Doc always gets first serve.  
(Once the measured portions are doled out, the food is available to each
until it's gone.  It's just the service that has to be done in the cage.)

I'm also greeting Doc first, letting him smell my hands and petting him,
then going to Pi to greet him and pet him, with Doc's smell all over my
hands.  

Pi's seriously confused at the moment, but he's not giving way lightly.  
He still tries to steal Doc's treats, even from inside the cage.  Rose,
the cat rescuer, says it may take a few months of this to get Pi to
understand that he's not The Boss.  

Doc has already stopped looking at Pi for permission to eat when I give
him food or a treat, so I have great hopes that at some point he'll stand
up for himself against Pi's attacks.  It'll take a while, though.  

So far it's been ten days or so of me playing musical cats, but it's
manageable.  At night and when I'm gone, Doc has the run of the place and
Pi is shut up in either the office or the living room.  There's a glass
door between the living room and the rest of the apartment, so they can
see each other then, as well as when one is in the cage.  

When I'm home, one is in the cage, and I switch them off so neither has
to stay in the cage for more than a few hours.  I think they like the
cage, because both of them go in there on their own if it's empty and
open.  Neither one likes to be *put* in there, though.  They have their
dignity, after all.  :-)

Anyway, for the time being, this is the status quo.  I'm going to have to
get a couple more small litter boxes.  I never thought four would be too
few for two cats, but I'm getting a little tired of shifting litterboxes
from one room to another when I move the cats.  That's the only hiccup so
far, though.  

So anyway, thanks to everyone who responded, I really appreciate all the
advice and help.  I'm still working on getting a screen door, but that
may be a while.  

If it's okay, I'll come back from time to time and let you know how it's
going.  If you think of anything else I could be doing, I'm all ears.  
(That's why I look so funny.)

Love to you all!

Chak