Two Southern California utilities should return at least $648 million to customers because of evidence of secret deal-making in a nearly $5 billion settlement following the closing of a nuclear power plant, a state agency said Friday. The Office of Ratepayer Advocates, an arm of the California Public Utilities Commission, said in a statement that private conversations between the commission's then-President Michael Peevey and a Southern California Edison executive gave the company an unfair advantage in negotiations over how to divide costs left behind by the now-defunct San Onofre plant.