Can I define multiple callback methods depending on the call method?

Check if your code looks like this because in the question you pass each method as separate function arguments (I have overlooked it earlier)

add_action( 'rest_api_init', function () {

    register_rest_route('my-project/v1/', '/form', 
        array(
            array('methods' => 'GET',
                 'callback' => 'GET_form',
            ), 
            array('methods' => 'POST',
                 'callback' => 'post_form'
            )
        ) 
    );
});

As you can read in documentation:

Parameters #

$args – (array) (Optional)

Either an array of options for the endpoint, or an array of arrays for multiple methods.
Default value: array()