DENVER — Police departments across the country are ordering officers to pair up after ambush attacks left eight officers dead in Texas and Louisiana, a precaution that could slow response times to low-level crimes and drive up overtime for already exhausted police. Some agencies that normally let officers patrol alone began forcing them to double up throughout their shifts, even during meals or other breaks during their shifts. Los Angeles police assigned members of specialized crime-fighting units to back up officers responding to routine calls.