It's a dumb limitation. The class system shouldn't even be here, no one uses it anyway, everyone just uses jobs. A huge point of this game is that one character can be all classes/jobs. By splitting smn and sch on one class it's proof that they won't get rid of the class system and just makes it so both those jobs can't be at their full potential if you want to play both.

And it will only get worse as more jobs are added to existing classes. Watch when/if drk becomes attached to gladiator