The standard implementation of jump in the PlayerBody does not support double-jump. It was written to try and emulate physically real behavior.
You may need to hack the function to take a second "allow-if-not-on-ground" argument defaulted to false, but with the ability for you to manually call to trigger a second in-air jump.
You may wish to put in a feature-request into the Godot-XR-Tools github ticket system.