Many historians have multiple fields. I know in the PhD program where I go to graduate school you must have a major field (what your dissertation is in) and three minor fields.

There are historians who dabble in everything, and they tend to be popular historians, HW Brands immediately comes to mind as does Douglas Brinkley. I'm actually in Historic Preservation/Public History, so my fields are quite varied depending on the current project I am working on. I have understandings of multiple fields but I haven't really mastered any one specific field, though I am currently making inroads into Film History.

Pick what you like. Just because you specialize in one thing doesn't mean you can't research and read about other fields as well!