Renting ADU on primary residence

@Carl  

I own a property with two separate mailing addresses.  I live in one building and rent out the other building.  The electric and gas utilities are metered separately, but water is shared.  I have one single mortgage for all structures on the land.  
I receive one property tax bill each year for the land and all structures on it. 

I have one property insurance policy covering both units but need to get a new one that separates the rental unit from the main unit I live in. 

 When it comes to entering my information in TurboTax, my plan is to treat the rental as a completely separate property and value the rental structure using the percentage of total square feet method.  How do I treat shared expenses like real estate taxes and mortgage interest?  
Thank you.