We’ve explained why it’s really not a great idea to update to the iOS 13 and iPadOS public betas right now — and the same goes for macOS Catalina. Not only is it unstable in its current form, but some things don’t work properly.
One of those is multiple iTunes libraries. As things stand, you will have access to only one. And if it’s not the right one when you first install Catalina, there’s nothing you can do about it.