If you switch the Multi off, the AC output will also be off. If you need to bypass power around the Multi, you need an external transfer switch.
Grid power normally has priority over battery power so leaving the Multi on probably does what you want without making any changes. What might require changes is to prioritize battery power at other times. If you wanted to totally ignore the grid during that period, you can switch the Multi to “invert only” mode.
You need to use a transfer switch or switch on AC1 input controlled by the Cerbo relay !
But only the AC2 out will be active when AC input is Off.
Xavier