For some reason they both ended up on top of the litter pan, and Nerissa was so squished (against the tub lid) it left small indentations on her belly. Any reason why they prefer being on top of that particular hide? Mona doesn't do that with her cave, so I'd totally get her another Exo-terra if they came any larger.
What about some kind of large flowerpot? Maybe she just doesn't like the square shape of the pan...