84:12 One may go out on Shabbos wearing two garments, one on top of the other, even if he doesn't actually need the second garment himself, such as if he's taking it to a friend. This is permitted so long as it's something that one might wear that way during the week, even if only once in a while. If one would never normally wear two garments like that, he may not do so on Shabbos because it would constitute carrying a burden. This is relevant in such cases as wearing two pairs of socks, or wearing a hat over a yarmulke.
84:13 If the local practice is to wear a plain belt with a fancier belt on top of it, one may also do so on Shabbos, even if he only needs one belt and is bringing the other one to a friend. If the local practice is not to wear two belts, he may not even wear the second belt for himself. Since one belt is sufficient, the second would be considered carrying a burden. In any case, one may go out with two belts when he is wearing layers between them, such as a belt on his pants and another belt on his coat. In such a case, one enjoys the use of both items.