i keep trying to mix their scents.
.............
Both are eating fine. Neither one seems too stressed out otherwise. I wonder if transferring some litter to each box will help too??
Mostly, give them time.
Has new kitty had a quarantine away from other animals before she came to you? New animals coming into the home should ideally have a ~2 week quarantine, either in isolation elsewhere like a rescue, or within their safe room in the home, before they meet resident pets in close proximity. That's just in case they're carrying something.
I would suggest keeping them mostly separate for now, but if possible able to see eachother. Feliway friends diffuser might help, it didn't do much last time I tried it, but others swear by it. I think it depends on the cats. Other calming products might also help.
If you're fairly sure your new baby isn't carrying anything that could be transmitted via used litter, then swapping a tiny bit of it might have one of 2 likely outcomes.
1. It works as you hope and they get used to eachother's smell.
2. Resident kitty develops toileting issues. If you're lucky, they resolve after a litter change, or at worst a new box and type of litter... If not, you're investing in an awful lot of enzyme based cleaner, to get poo and wee out of your carpets, bed, shoes etc.
A safer option might be to make both kitties smell different, but the same as eachother. Pet-safe wipes are a good option, unless you think you can manage to bath them both with pet safe shampoo? I've got some cat wipes that smell lovely to humans, but I'm not sure the cats appreciate the orange scent as much as I do.
Mutual play, and mutual treats might also be useful for desensitizing them to eachother.