You should’nt do that.
In Batocera, you configure your controllers in the EmulationStation menu accessible through the Start button of your controller or by the prompt telling you to configure your controller.
Then in Mame Select button is used for coins and star button for start.
So select on player 1 controller = coins 1 and start on player 2 = player 2
Except if you want to map very specific controls for one game or systems, you should never modify crontols through retroarch or mame service menu