Psychological Warfare with Cats

"When my cats aren't happy, I'm not happy. Not because I care about their mood but because I know they're just sitting there thinking up ways to get even." - Percy Bysshe Shelley 
 
I have 2 cats.
 
I'll make formal introductions another time, but they are named Sam and Kate.  Both are female, about 10 years old and I adopted them from a friend of mine.  Fantastic cats with definite personalities.
 

Kate (left) and Sam (right)

Sam has been losing weight for a while, but I didn't really think too much of it at first because she was overweight.  Then she started to lose too much weight and I found out she may be closer to 16.
 
Off to the vet we went.
 
Hyperthyroidism.
 
According to Mirriam Webster Online: excessive functional activity of the thyroid gland; also: the resulting condition marked especially by increased metabolic rate, enlargement of the thyroid gland, rapid heart rate, and high blood pressure

She needs special (read expensive) food.

Have you ever tried to feed one cat one kind of food and keep the other cat from eating it?

Yeah.

It's ok for the other cat to eat this special food, but as it is very expensive, I'd rather she stuck to the old food.

I started off mixing the new food with the old food.  She would pick around it. (point to the cats)

I tried the "you'll eat when you're hungry" method.  After three days of looking at her recently acquired thinness, I gave in.  (another point to the cats.)

Sam couldn't jump very high - not the most graceful of felines - so I fed Kate her old food on the bathroom vanity.  Sam was hurling herself up there to get at the other food.

I tried the canned version (even more expensive).  Sam would eat this, sort of, but Kate was jealous, shown by increased aggressivness in their play.  (point 3)

Of course, Sam didn't like it in a chunk from the refrigerator.  So I'd have to mix it with water and feed her on the couch next to me to keep Kate away (yet another point)

Kate was even more jealous so I bought her some cheaper canned food for her.  (how many points is that?) 

Funny, I seem to be losing this battle.

At this point, Sam has decided she doesn't like the canned food either and she wants Kate's food while Kate is avid to get Sam's food.  (I can't win.)
Finally, a PLAN of ATTACK.  I continue to feed them separately but I give them the SAME(expensive) canned food.  I check back and Kate is happily eating Sam's food and Sam is happily eating Kate's.  (Finally, a point for me!)

The next day, Sam is not eating again and Kate is eating two servings and getting fat!

Cats 8
Me 1 (ish)

Sigh.  I surrender.  Back to the vet to discuss alternate options.