This can be either a macro or a button on your characters, it doesn't matter. For some reason though when I put multiple /useitem lines in a macro it doesn't seem to fire properly - so you may want to keep that in mind when setting this up on your characters.
Depending on what clicky you're planning on using determines on what binding you want to use/where you will want to put it at on your bars. For example, I would recommend putting "Miniature Horn of Unity" on any character that does not have a Twincast Macro in that spot. Or Diplomatic Papers on any character who does not have a Protection Macro in that location.
For other clickies like Rage of Rolfron or Bifold Focus of the Evil Eye (Twincast clicky) you may want to give them their own slot/keybind instead of putting them in the position of something else. Basically the idea behind this is to make use of available slots on your characters.