Inappropriate peeing and stalking

We got a new cat in October after Summer was an only cat for a long time. Summer has a history with UTIs, so when she peed outside the box in December, I took her to the vet, and sure enough she had a UTI. They said the lining of her bladder wasn't healthy, she didn't have any crystals, and there was some bacteria. She completed her round of meds, and I took her back Monday for a re-check, and she's in perfect health. However, she has peed outside the box since. She is also on cosequin now as a preventative since she has a history with UTIs.

I think it may be behavioral because I've seen Chevy "stalk" Summer while she was using the litterbox. One time I seen him attack her while she was trying to go. She just left the litterbox, and went to the 2nd litterbox. Chevy followed her and stared at her while she went from a chair about 2 feet away. I use unscented litter, have 2 boxes, scoop twice a day, and all that stuff. Both litter boxes are on opposites sides of the apartment, and in open areas. That's what makes me think this is the problem.

We are not home during the day, and both have full-time jobs. Is there anything I can do to prevent this from continuing?

I have a similar problem. The only difference is that Frankie was peeing outside the box (when healthy) even before we got WIckett. He also stalks & harasses her when she's using the litter box. We try to reward her as much as possibly when were home and we also try to help her maintain her privacy but these are all things that we can only do when we're home. Have you tried a pheramone diffuser or spray like feliaway? That might help Summer feel more calm and might even get Chevy to calm down and leave her alone more as well.

first off get more then one litterbox. You should have one per cat and one extra. you can also plug in some feliway or spray it in your home.

I haven't tried feliaway. I will buy some though! I did try adding a third litterbox, but they didn't use it, so after 2 weeks, I gave up on the idea. There aren't many options living in an apartment, so I couldn't try somewhere else.