By John Poirier WASHINGTON (Reuters) - U.S. regulators took over two banks on Friday and sold them to Mutual of Omaha Bank, the sixth and seventh bank failures this year as financial institutions struggle with a housing bust and credit crunch. ADVERTISEMENT Two weeks after the Federal Deposit Insurance Corp seized IndyMac Bancorp Inc (Other OTC:IDMC.PK - News), the Office of the Comptroller of the Currency said it closed First National Bank of Nevada and First Heritage Bank NA of California. First National had total assets of $3.4 billion and $3 billion in deposits while First Heritage had assets of...