If you use it on the same devices, wifi networks, locations, etc as your regular Facebook/Insta then you’re not fooling anyone. Meta still knows it’s you, and will happily link the data from the shadow account back to your regular accounts. They can track you across the web even when you don’t have an account at all, because all of those “share to Facebook/Insta” buttons tell them that you visited the page and loaded the button. You don’t need to be signed in for those buttons to load. And you think a shadow account is going to protect you?