The U.S. Constitution establishes three separate but equal branches of government[…] https://www.uscourts.gov/about-federal-courts/educational-resources/educational-activities/first-amendment-activities/us-v-alvarez/separation-powers-action-us-v-alvarez While the constitution speaks of “separation of powers,” the ‘but equal’ phrasing isn’t uncommon. But you’re correct that the phrase is better (and more accurately) known for the segregationist doctrine.