LuCiel is a special character, as despite being classified as despite being one unit, they are still considered two separate characters. This makes buying clothes/idles/sit motions for them way more expensive then any other character. Some outfits are already close to $40, now factoring LuCiel, it's $80....that's just unfair.
In all cases save for Ice Burners in the Item Mall, I suggest combining both Lu and Ciel's Cubes into one. For example, for event dungeons, opening costume cubes would give BOTH Lu and Ciel their respective costume pieces. When buying costume packages in the Item Mall, both Lu and Ciel will be condensed to one package, and that will cost the same as everyone else. Sit Motions will no longer be segregated; buying a motion will give it to both characters, just like the weekly quest's hot springs sit motion. Finally, ONLY Ice Burners given from events like Anniversary will be given to both characters. The 3rd Job Promotion AND Hyper Punch Master events already do this.
The reason why I left out in season Item Mall Ice Burners is because with expensive, valuable pieces such as the rings, giving LuCiel two would only provide an unfair advantage. Players would be given two rings, only of which one would be used, and could sell the other off. Event Ice Burners don't count because they're usually account bound anyways (speaking of, let event ice burners be bank shareable).