Medication doesn't work that way. It doesn't cure behavioral problems, it can aid them, but not alone. Xanax is for anxiety and Compulsive Disorder (animal version of OCD).

Have you worked with a reputable cat trainer? Behavior modification can be paired with medication, if appropriate, but medication isn't to take place of BMod.

Let me know if you need a referral to a trainer in your area.

If you have one or two children and then decide to foster another child would you put them on anti-depressants if they all didn't get along ? Why are we always reaching for the pill ...
Cats are territorial and are upset by any changes in their routine. The original cat or dog must be respected and introduced to a new roommate deliberately, slowly and even then jealousies may still exist. Have you read any books are how to make introductions ? Anita Frazier writes excellent cat books.
Would you like a complete stranger to be sharing your bathroom and eating dinner at your table ? That's how your cat feels.
Resentments burn long if they are not addressed.

I have not had a lot of luck with 'training' cats to get along, and behavioral modification in cats can be frustrating (many cats seem to want to be the only ones in the home and misguided attempts to get them a 'friend' often end in disaster). Still ,it is certainly worth trying.

I have several clients who own cats and have one 'bully'. Sometimes fluoxetine works well for these cats (I have actually had very good success with this medication when used for this purpose in cats). This is the generic for Prozac and it's now sold as Reconcile specifically for animals. This drug works very well, too, for anxious cats that tend to have behavioral urination problems (way better than any other medication I have tried). I know medications are not the cure all for behavioral problems, but some do help a lot, often saving a pet from being sent to the pound for euthanasia.

Still, in some homes, there remains some tendency for fighting and a constant tension may exist... and some of these owners elect to separate the cats into a few permanent groups if possible.

Clinical Applied Animal Behaviorist or even a Veterinary Behaviorist. The $ is worth it to save a life and keep the home.

To start with Go get a second cat litter box or a third if needed, Multiple cats require multiple litter boxes,

gogwmos has been dealing with this for 1 1/2 years and I believe posted on this site around June of this year. IMO, before trying to control Cali with drugs, and it appears she has no intention of changing, I'd start looking for a good home for Amy.
I believe such ongoing stress will eventually cause some type of illness in Amy and from your description, she's too sweet to lead such a fearful life.
And Cali will be much happier..............