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