Merge permissions from multiple Drupal roles

First specity the new role which will be the source and destination for the merge

$source_role_names = array('blogger', 'editor')
$dest_role_name = 'merged';

Fetch the rid from the names

foreach ($source_role_names as $name) {
  $rid = user_role_load_by_name($name)->rid;
  $source_role_ids[$rid] = $rid;
}

Create the role if necessary.

$role = user_role_load_by_name($dest_role_name);
if (!$role) {
  $role = new stdClass();
  $role->name = $dest_role_name;
  user_role_save($role);
}

Get all the permissions from all source roles

$result = db_query('SELECT permission FROM {role_permission} WHERE rid IN (:rid)',
  array(':rid' => $source_role_ids))->fetchAll();

foreach ($result as $record) {
  $role_permissions[] = $record->permission;
}

Grant permissions for the new role (merged)

user_role_grant_permissions($role->rid, $role_permissions);