The Occultist archetype "Esoteric Initiate" grants a book as an implement that works for any two schools of magic. This implement doesn't have the additional language from Battle Host which states that invested mental focus need be split between schools.
Given that, I'm wondering if the Battle Host split focus rule is meant to be clarification of a universal rule for occultists, or a specific rule exclusively for the battle host.
Beyond that, as it grants a book as an implement, is it possible to use that same implement for divination if it was used for two other schools when you received the implement.
It's hard to judge the intent of the archetype's ability as either reading of the rule removes something from the class, and its delay of Panoply access is also significant.
If there's an faq regarding Occultists in general I'd like to see it.