I also have another concern,
1) Post DNS re-routing from primary ePO to secondary ePO, the agents communicates to secondary server by downloading new sitelist.xml file.
2) What do I need to do, to make the agents communicate back to primary ePO server from secondary, Do I need to re route the DNS again from secondary to primary, or just shut down the secondary server by bringing the primary online?
You don't need a second ePO server here, you want a Agent Handler here. Setup files are with ePO, its get added into ePO and added to the sitelist for all the agents. This will enable all the agents to automatically fall over when the Primary ePO server is offline.