It could be a dominance thing. It could be an intimidation thing if Mika doesn't want to risk being cornered in a litterbox by Midnight. It could be a stress thing if Mika doesn't appreciate having Midnight in her territory. It could be an illness thing if Mika has developed a urinary tract infection, which can result from stress.

Why would you even consider getting rid of Mika when you caused this problem by bringing Midnight into the household? Mika has to pay for your mistake?

Take Mika to the vet to check for UTI and/or urinary crystals.

Get another litterbox. Place the litterboxes in different rooms, NOT in corners where Mika may feel trapped if she tries to use one. Try both covered and open boxes.

Buy some Feliway or Comfort Zone to use in your home to help calm your cats and hopefully help alleviate the inappropriate urination.

Buy some Rescue Remedy and give it to Mika to help relieve her stress.

Buy some Cat Attract and use it in your litterboxes.

Give Mika LOTS of extra loving attention away from Midnight so that she knows she is not being displaced. Do NOT allow Midnight to intimidate or harrass her in any way.

If the previous suggestions don't resolve the problem, you may want to discuss the possibility of putting Mika on an anti-anxiety drug as a last resort.

Mika needs your love, reassurance, understanding, and patience. She is responding the way a cat under stress responds. It's your responsibility to do everything possible to alleviate her stress, and "getting rid of her" isn't the answer.

Laurie

I just want to reiterate everything laurief has said. Very good advice.

Since you mention bringing another cat into the house, that may well be the problem. Some cats are terrified of being cornered in the litter box by another cat. Make sure your litter boxes have adequate space for a cat to escape. Get rid of the covers for now, if you have them. Move them away from walls/corners.

As far as antianxiety drugs, they are a reasonable option. No need to "get rid of" your kitty if a little Clomicalm or Prozac will do the trick. They're not very expensive, and your cat may just not be adjusting well to the new addition. Obviously, try the other suggestions first, but there's no shame in giving a cat a little pill to help it deal with "life". Cats are notoriously sensitive to changes in their environment. Don't get rid of your cat because she is upset with the new cat you brought home. If she needs some medical help to deal with the change, it's not that big of a deal. I don't like cat pee, so I'd probably medicate the cat right away (provided there are no physical problems), just so I don't have to keep doing laundry. Then, if everything calmed down, I'd wean her off the meds to see if she was okay on her own, which, in my experience, they often are after some time to realize it's not the end of the world.

I'll also second the use of medication for anxious or overly dominant cats. Obviously, try other measures first, but if those don't work well, try Clomicalm or Prozac. Our male is on Clomicalm, and it's worked very well for him.