Why do you want a backup MX server? It doesn't make mail delivery more reliable.
Internet standards specify that mail senders (ie sendmail, exim, postfix, qmail etc) ALWAYS queue & retry. So if your server is down, the email will be queued at the sending MTA and retried later.
Having a backup MX can actually expose you to more Spam. Spammers usually send to the backup first, relying on it not having as good spam checking - so it can actually increase your spam load a LOT.
In fact, backup MXes make you more likely to lose email as you may get an event over time which causes email sent to the backup to get lost. This is actually more likely than you'd think over a few years as backup MXes tend not to get checked regularly.
So, in summary, don't go there - it won't add reliability, and it will add admin complexity. Perhaps if you're running a server farm, and you know what you're doing, it may help; but for the average cpanel hoster it's a waste of your time.