Can I write two cron( hook_cron() ) in one module
No. You can implement hooks once per module due to the namespacing of the hook (mymodule_cron). However you can put conditions within the cron hook to do different things.

You might be looking for this instead: https://api.drupal.org/api/drupal/modules!system!system.api.php/function/hook_cron_queue_info/7.x