Can one professor hold more than one endowed chair? Are there examples of one person holding more than one endowed chair at the same university?
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 (for example, History of Science and Psychology), they were usually listed as the:


Dunning-Kruger Professor of Psychology and History of Science


or alternately, if the endowed line (the DK-Professorship) had been bequeathed specifically to Psychology for example:


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




So the short answer is: no.




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 

Charles Warren Professor of American Legal History at Harvard Law School
Carol K. Pforzheimer Professor at the Radcliffe Institute 
[Harvard U. FAS] Professor of History



I personally think that this exception proves the rule since the three professorships are at nominally separate entities (HLS, Radcliffe I, and Harvard U FAS) that are under the singular Harvard conglomerate umbrella.