If the ethicist's utility function is broad enough in scope to subsume (or at least recognize) that what is good for the economist is ultimately good for the rest of the system (if that were the case), then there might not necessarily be a need for distinction of utility functions.

The subsumption operates in the other direction. Utilitarian ethicisists can alway be modelled as rational agents with a specific set of preferences. Purely selfish agents can only be described using the language utilitarian ethics in the counter-factual world where purely selfish behavior gives perfect utilitarian outcomes.