I want to install a backup mail server, at this moment I have the main server running and I have the old server of mail disconnected of the network, my question is if I can maintain both servers connected and configure the dns so that the old server works when the present one has some fault, and update both, thanks for your comments
This is quite easy to do and was in the past regarded as "good practise", but these days I would suggest that you didn't bother. Having a backup mail server presents an additional target for spammers to try to get mail through to you, and you will most likely start generating "backscatter" (bounces for mail with forged sender addresses), when mail is delivered via the backup server and rejected by the main server.
The mail servers at the sending side should queue mail up for at least 5 days, repeatedly trying to send mail to you, so unless you expect your main server to be down for that sort of period, a backup really is more trouble than it's worth IMHO.
Paul.
