This actually might be less of an issue about privacy and more of an issue about resource guarding.
This can be a problem in multi-cat households and often centers around key resources like food, water, litter boxes, and even prime toys or napping spots. Resource guarding can be easy to miss because it often doesn't look like aggression, so it's great that you picked up on this. Usually the cat guarding the resource hangs around it (or even by the doorway of the room that has it), follows the other cat to it, stalks the other cat as they use the resource, or chases the other cat after they've used it.
So while it doesn't appear to humans to be aggressive, it's likely your younger cat is policing this important resource and it's making your older cat feel threatened. But it's important to remember that both of your cats are feeling resource insecure and it's likely stressful to both of them. The younger one has an insecurity that is driving it to guard the resource and the older one is now also feeling resource insecure due to the guarding behavior of the younger cat. Keep in mind that your younger cat's insecurity might stem from prior experiences before you got her - so even if the resource is abundant in your house she might still feel the need to guard it based off her history.
Ways to help alleviate this are increasing access to the resource and interrupting the behavior when you see it.
- When you think of increasing access I would think about this in terms of numbers, but also location as well as entry/exit points. If it's happening around both boxes, adding a third could work. If it's happening around a box in particular consider adding another box to that specific room or repositioning the box so your policing cat is less likely to be able to ambush the other one and the other one has multiple means for escaping. If it has a lid you might consider removing it as it will vastly increase the number of exits your older cat has.
- Try distracting your younger cat with toys or another form of engagement when you notice your older one going toward or using the box.
You know your cats and their behaviors best so I'd suggest doing some internet searches around 'resource guarding behavior in cats' and see if what's described there matches what you're seeing. There are a fair amount of online resources and tips/tricks for addressing it. This is one specifically about litter boxes.