There is no "magic button" that magically and instantly combines the two - - because it requires a human being looking at reach record to really determine what needs to be combined, and on what side of the fence, and based on what set of rules.
any merge process would really require going record by record.
Keep in mind in Ancestry - you are the owner of your own private database
in FamilySearch - you are one of a million collaborators who all work on the same public database - so you have to be very keen on the fact that anything you do in FS can and will impact other users and their "view" of the database.
this is a link you may wish to read: