I had personally never heard of anyone occupying two endowed chairs at the same time at the same university. Even if a person had two appointments in separate departments at the same university, they were usually listed as the:

* Dunning-Kruger Professor of Psychology and History of Science

or alternately:

* Dunning-Kruger Professor of Psychology, Professor of History of Science

if the endowed line (the DK-Professorship) was bequeathed specifically to Psychology for example.

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But that being said, there are always exceptions. A little digging around at universities that have separate sub-units revealed that, at Harvard there is a person in History who holds two endowed chairs and one regular position:

* [Annette Gordon-Reed][1] 
 * Charles Warren Professor of American Legal History at Harvard Law School
 * Carol K. Pforzheimer Professor at the Radcliffe Institute 
 * [Harvard U.] Professor of History


  [1]: http://history.fas.harvard.edu/people/annette-gordon-reed