The phrase "multiple airlines" means more than one airline. It refers to the situation where there are several different companies providing flight services. Full definition
I go out of my way to maintain my status with multiple airline alliances for the perks and conveniences it offers! (adventographer.com)
You can filter through multiple airlines by checking the boxes on the left. (travelcodex.com)
Because if you don't have set travel plans for a future trip, these transferable points give you the option to choose from multiple airline partners when you're ready to book. (travelpointsguide.com)