The Witch is a very different witch, although she's pretty familiar with a certain bathhouse herself. Having gone there on vacation when she was younger, she saw how things were done and tried to build upon them with her own island! So Chihiro would definitely be free to notice similarities in island layout (such as the bathhouse), business ethic and so on or even assume she was Yubaba (if. In disguise), but they're different people.

As far as direct contact goes, everyone meets her when their contracts are signed, but this is generally handwaved because it's your entrance -- she'll definitely be an interactable NPC in the future, but not in the first month of gameplay. Way too busy. It's Tanabata, after all.

I hope that makes sense!