Ok cat man  before I go to far and someone else says it right off the bat.
Are you willing and able to take the cat back to vet if needed.  You may
have to but before we get that far  here is a little bit below

You went to he vet  what was the diagnoses, what was the work up done.
How long ago was this?
Is the cat in any pain when you pick it up or you notice and problems when
it is urinating?
If it was a while ago for the vet  you will need to take a sample of the
urine and stool to the vet for diagnose of possible infection. You may need
a complete history and physical, stool / GI workup, workup for
polydipsia/polyuria

You must treat/correct medical problems first. Behavioral problems can only
be diagnosed in a healthy cat

You broil them chicken are you taking the proper cleaning and cooking of the
chicken ( as  I said nothing meant by just asking)

Some basic information for us out here to help out

Are they inside or outside cats?
Pure breed Siamese?
Are both fixed?
Are you sure it is him doing it and not the other cat urinating and the
other cat being attracted to the scent?
Is there any stress in the house right now?
Anything that would cause this behavior such as new room mate   moving the
furniture all around, new job schedule  things of this nature if you get my
drift?
Is the firball on medication for it's digestive problem?  If so what is it?
Have you removed any carpet or area the urine was present?  If not have you
clean the area and how did you clean it?
How many cat boxes do you have and what type of litter are you using and
where are the boxes at?
Have you changed the cat boxes locations to where the firball is going at?
are you making sure you have a clean box for them ( sorry nothing meant by
this just asking to rule out )

We will start with this question list  I am sure some will point out if I
missed something.  A

http://www.fanciers.com/cat-faqs/behavior.shtml
http://www.fanciers.com/cat-faqs/general-care.shtml

Matthew

Did you take him to the vet for a urinalysis since this started? That
would be the first thing to check.

It could also be stress or litter box aversion, here's a decent article
which covers some of the issues:
http://www.peteducation.com/article.cfm?cls=1&cat=1310&articleid=158
You can also google inappropriate urination and find lots more that can
be helpful.

I agree with 5cats, the cat needs to be checked for crystals or bladder
infection. He may be under stress from not feeling well, and that could
cause a flare-up of a urinary problem.

For the digestive problems, has the vet suggested it could be food
allergies? Our cat had suspected food allergies or IBD, so she put him
on a prescription diet. The brand was IVD, and there are very few
ingredients in it. It uses a novel protein like duck, and one starch
like peas. Many cats are allergic to wheat or grains.

Hope you can find the cause for both. You and the vet might have to
become detectives and rule out different things until you find how to
help your cat.

Take care,

Rhonda