You can easily share you Instagram Story post to your Facebook Story post.

First, make sure your Instagram account is linked to your Facebook account. You can do this under setting/linked accounts.   Once they are linked, post a story to Instagram story. You should see a facebook icon when you view the story. Click that and it will share it to Facebook story.     If you want this to happen automatically, press the 3 dots for more options and choose story settings. On the button, you can turn on share to Facebook by default.