As I learned from Seth Godin some time ago, you don't have to be any more expert than knowing more than the reader you're creating content for. I create content on multiple subjects as I learn on multiple subjects. Some of them overlap.
In a world of billions, the concept of brand is what a person makes it. As a content creator, my brand is content that is either useful (my books) or informs (my writing).
Multiple audiences are not impossible to have. It can be one audience for one subject I'm known for or more that I'm known for.
In my offline community I'm known for my expertise in leadership, volunteerism and non-profits. Online I'm known for other things.