Second PhD vs PostDoc - Math vs Theoretical Physics
Anyone will hire you provided you can convince them you are better than the other candidates. You already have a PhD in Maths, you have to show them you have the skills and knowledge to complement their baggage and help solve their problems.

In the Mathematical Physics department in my undergrad university (Spain), there is at least one postdoc that is mathematician with a PhD in Economy, and two physicist readerers that works in Applied Mathematics.