I had personally never heard of anyone occupying two endowed chairs at the same time at the same university. Even if a person has two appointments in the same university, they are 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 is specific to Psychology for example.
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] Professor of History