The usage is as follows
The main part has a backup. The main part and its backup each has a high side swith. The main part and its backup have a common Load connected to both high side swithes.
When the signal MAIN_EN is active, the isolated DCDC of the main part output power and that of the backup is disabled. If the controller in the main part power domain turn on the swith, the load is powered. Will the current flowback from the load to VOUT and finally to the VBB of the swith of the backup?