Litterbox issue with my new kitten

I'm seeing some odd litterbox-related behavior from my new kitten, Shiva. She's just under four months old and a bengal. We've had her about a month now. We also have a 1-year-old calico, Tika. Shiva follows Tika around everywhere, like a little shadow. It's adorable, but this "shadowing" also includes when Tika goes to use the litterbox. Generally Shiva will hop in as soon as Tika's done and do her business. This morning, however, while Tika was in the box, Shiva pooped on the floor, right next to the box - while Tika was in there. I've found poop on the mat outside the box twice before, but at the time assumed it had just gotten kicked out as they were trying to bury it. 

We have three litter boxes in the house (which we clean twice a day) so I don't think this is an issue with Shiva not wanting to go in a dirty box, or there not being enough boxes in the house. Any idea what this was about? Is Shiva trying to like, claim territory or something? Or could this be a medical issue? Should I switch to larger litter boxes so both cats can get in there at the same time?

Thanks.

Hmmm...it could be a preference for that particular box. I would try adding another box next to the popular one, or trying a larger one. It sounds like Shiva's trying to mimic her new idol, but just being a bit to young to hold it, so a second box or bigger one might help.

Does this tend to happen at the same box? If so, there might be something about the placement of the box that makes it more appealing than the others. If you can figure out what that is, there may be a way to put the others in equally appealing places.

You can check out this article for ideas about what might be making one box appeal more than the others:
[article="22302"]Litter Box Location Secrets  [/article]

That does make sense. Every time this has happened, it's been by the same box - which happens to be the one that's furthest away from the "main area" of the house. I'm going to try adding a second box to that corner, and maybe re-positioning one of the other two. Thanks for that link, it was helpful!