How often do Ph.D. holders hold faculty positions in different, but related, fields from their Ph.D.? This is something that I thought about a while back when reading (I forget where) about someone who held a Ph.D. in chemistry, but focused on physical chemistry throughout his Ph.D. years and actually became a physics professor. 

I was wondering how often this happens. For instance, do electrical engineering Ph.D. holders with very physics-oriented research become physics professors ever? What about vice versa? Or what about, for instance, applied mathematicians specializing in cosmology. Do they ever become physics professors?

More important question here: does this open up the possibility in which one can apply for faculty positions in multiple departments, thereby increasing one's chances of a faculty position? Or does the interdisciplinary nature of it actually limit the candidate's faculty choices?

Note, when I say someone is a professor in another field, I mean, for example, they received a Ph.D. from a department of chemistry and were subsequently employed by a department of physics.
For interdisciplinary researchers, this sort of thing can happen fairly often.  For example, if you are a chemist working on ecological applications, you might well have started out in a chemistry department and end up in an ecology department, or vice versa.  This happens even more so when dealing with disciplines that may be categorized differently by different institutions: for example, is work on medical devices part of biology and the natural sciences faculty or part of electrical or mechanical engineering?  Generally, however, if it's going to happen, there is at least some logical trail that you can see how the faculty member got from point A to point B.