For old version - symantec antivirus. I would setup two antivirus Server for redundancy.
For SEPM version.
Still can use replication as Primary-Secondary system by installing it on two computers. But will not allow a secondary server to become primary and to have itws own replication partner.
If don't use replication method, then backup database and backup related information into document. It can restore exact copy of the current one on new SEPM.
This method just needs one computer.
I would like to ask which method will all of you choose? For trouble free, I would like to have exact copy of current one on new SEPM in case of disaster. But seems unsafe to have one antivirus server. Please advise. Thanks.