Gutenberg registerFormatType with Multiple Classes

As we can see from the registerFormatType function source code, only letters, numbers, “_” and “-” are allowed in the class name.

if ( ! /^[_a-zA-Z]+[a-zA-Z0-9-]*$/.test( settings.className ) ) {
    window.console.error(
        'A class name must begin with a letter, followed by any number of 
         hyphens, letters, or numbers.'
    );
    return;
}

So you cannot separate two class names with a space.