How to add a second user to the iPad Air?

Neither iOS nor iPadOS support multiple users. And if you switch back and forth with AppleIDs in iCloud you’ll end up merging each other’s data on both AppleID unless you erase the device between each change of AppleID.

Sorry, not possible.

Outside of educational environments, iOS/iPadOS devices are essentially intended to be single-user. There is no provision to create additional user-profiles for multiple users.