Sharing game progress across 2 consoles (belonging to 2 separated parents)

I think extended is wrong.

The true and first meaning of the account/game sharing was to a house share like 2 brothers with 2 consoles and for the parents doesnt need to buy 2 xbox live golds or 2 games. And not to people share with all their friends.

Doesnt mix the family settings with account sharing on Xbox, is not the same thing and not the same end.

You would need at least 2 in order to share across 3 consoles.

As ChiefJoboo mentions, there is no sync across the games.

They are 2 separate games, each with their own achievement lists as well.

Also if you're using 2 separate consoles. The other user needs their own copy of the game for it to work. You can't share the same game on two consoles simultaneously.

Gold can only be shared on the console you have set as your Home. It cannot be shared across 3 separate consoles. In that type of situation you would need at least 2 subscriptions. You set one of the consoles as your Home, then you can sign into another
one and still use gold as long as you're signed in, then one of the kids would need gold on their account and sign into or set the 3rd console as their Home.