Glad to hear you will be separating them, they are both very cute
It may not be related to cohabbing but it's difficult to rule out other causes of her slow growth with another lizard living with her.
You may not see obvious bullying but animals can have subtle behaviors that we may not notice. Once dominance is established, all it may need to be is body posture, eye contact or just the simple presence of the dominant lizard. It's good they are fed separate but they can still be competing for other resources (best basking spot, best sleepig spot, best view, space) and the submissive lizard can end up in a long term state of stress from her roommate.