ACF already does validation and you can validate one field against another field using those filters. Here’s an example
add_filter('acf/validate_value/name=password2', 'my_validated_password_filter', 10, 4);
function my_validated_password_filter($valid, $value, $field, $input) {
if (!$valid) {
return $valid;
}
// field key of the field you want to validate against
$password_field = 'field_0123456789abc';
if ($value != $_POST['acf'][$password_field]) {
$valid = 'Does Not Match Password';
}
return $valid;
}