Only items purchased outside the game are available on both servers. If you buy something from the ESO website, or from 3rd party websites and then apply the code it will be available on both servers because there's no way for the game to know which server you'll want it on.

If you buy something from the crown store while logged into the game it won't be available on the other server because there's no way for it to update the other server with your purchase.

(Also if you mean outfit slots used to save your own combination of armour skins then those are even more limited - they're character specific so even your other characters on the same server won't have access to them.)