Family Sharing

If two people are using the same Apple ID (not usually a good idea and can be a quite bad idea) they are effectively seen as the same person using multiple devices belonging to that person and there is no need or way to use Family Sharing between those two people.

If you both are using the same Apple ID, you are already sharing purchases, and data. For Family Sharing, each member of the family must have their own Apple ID.

See if anything in the article(s) below helps.

Family Sharing - Set up.  *