National and Alamo are owned by the same parent company. Many (if not all) US airport locations are run the same way as Las Vegas, i.e. the same rental location with a shared shuttle bus. Each brand has it's own rental counter. In some airport locations the cars are all grouped together, in others they're separated by brand. National caters more to the business traveler (higher prices, faster service), while Alamo caters to the leisure traveler (cheaper prices, slower service).