According to Young Yang over at the Office 365 Community Forum, we can combine Business Premium (BP) and Business Essentials (BE).
The TechNet Official statement says it cannot be combined, so I Guess its not in general available for customers buying services through agreements.
How does it work?
Create a BP Trial tenant and BE will be available among the Plans to purchase, making them bought available for use in the same tenant.
The only difference between BP and BE is a Plan named Office 365 Business, which I Guess includes Office Pro Plus. Making them difference the same as E1 and E3. And those are available to combine, so BE and BP should be combineable.
Right now I believe the TechNet article shouldn’t say BP and BE are two different Service Families, like the Small Business and MidSize Business was.