Hi Manjot,
Thank you for posting in Symnatec community.
It seems you have to configure additional SEPM either as a replication partner or failover/load balancing.
No need of additional license in this case. Check this article to get more info:
Can a single license be split between multiple separate Symantec Endpoint Protection Managers?
http://www.symantec.com/docs/TECH164392
I configure the setting that when main server restart/disconnect client communicate to new server, will it back again when main server install?
--> Yes
How to install?
--> You have two options.
1) Failover & load balance server:
About failover and load balancing
http://www.symantec.com/business/support/index?page=content&id=HOWTO26809
OR
2) Replication partner
How replication works
http://www.symantec.com/docs/HOWTO55328