It happens when you level up and select a subclass (E.g clicking on the Oath of the Motherland) but cancel the level up. When you attempt to level up again you will have unlocked that subclass's abilities (Motherland) and can still choose a different sub-classes powers. This allows access to the spells and powers of both.
I tested this with the Paladin(Had a Motherland/Tirmar Paladin) and the Ranger (Had a Marksman/Hunter Ranger). I assume that you can do this with the wizard, but I do not believe you can with the Cleric as they get their sub-class at level 1 and not through a 'Level-up"
Don't mess with the subpar.