Startup Parameter

Race Element has got a command line parameter that allows you to start it minimized: /StartMinimized

Starting the app and ACC using one batch file.

You could make a batch file that for example starts race element in minimized mode and launches Assetto Corsa Competizione through steam.

START "Race Element" "PATH_TO_FOLDER_OF_RACE_ELEMENT\RaceElement.exe" /StartMinimized
START steam://rungameid/805550