The U.S. Air Force is negotiating with Boeing [BA] a multi-year procurement buy for the company's F-15EX Eagle II and the possible opening of a second and third production line in St. Louis for the fighter, a top service official said on Tuesday. "There is ongoing conversation with Boeing to getting to [building] two [F-15EXs] a month," Air Force Lt. Gen. Luke Cropsey told the Senate Armed Services Committee's (SASC) airland panel on Tuesday in response to a question from…