DUBAI—Qatar Airways and Emirates Airline said they joined forces to negotiate with Boeing Co. for almost $100 billion in deals they signed for the new 777X passenger jet, a first for the carriers that further underscores the shift in power in the aviation industry to the Middle East.

The combined effort is the first time the two Persian Gulf carriers—the biggest airlines by passenger numbers in the region—have collectively pushed for better terms from an airline manufacturer, sharing information to ensure the new plane meets...