This is a good question. So many people today sell books or become famous on TV or radio as doctors, but their doctorates aren't in what they're talking about. What I've noticed with many people who have more than one specialty is that they put a list of their degrees after their names, but if you're acting, say, as a pop psychologist but your degree is in history, well, you oughta not put that PhD on your psychology credentials, right?
I would say that his/her shingle should have Dr.______ PhD