It sounds like primary/secondary combination spells or class combo specific spells. For example: A warrior/monk would have skills a warrior/necro could not have acess to.
Sounds interesting, but you have to realize that there are already some skills that are almost built for a secondary, and also some that mix classes well. For example, if you are a ranger + elem and have a bow with elemental dmg, you can boost the dmg by using the elem spell conjure ____ (bows elemental dmg). Also, if you are a W/E using a hammer, you can knockdown and hit aftershock for the bonus dmg. While the spells aren't for a specific coupling of classes, some come very close. At the very least, there are certain moves that have synergy with another class instead of your charachter being 2 completely separate classes.