Dual failsafe in Copter

I’m trying to figure out the drone behavior in case of multiple failures
Such as GPS FS and then battery FS, when the battery failure setting is RTL
or RC FC and then GPS FS …


Every failsafe event takes its specified actions regardless of what other failsafes may have taken place before it. The wiki does a good job explaining what each failsafe will do. All of them are listed here: https://ardupilot.org/copter/docs/failsafe-landing-page.html

FYI there is no such thing as a “gps failsafe”. Losing the GPS would trigger the EKF failsafe, just like losing a compass would as well. So that’s why you don’t see something called a GPS failsafe.