Secondary DNS + MX Backup

I'm new to i-mscp so don't take my word as holy truth

If you want just a second IP that redirects to your imscp server, you could just use one of the public IPs of your separate network. This could work if your second network is in another geographical site with it's own public IPs, or if it is located in your primary site but you have a second line/adsl with public IPs. If you have just one adsl/line this configuration has no sense at all.

If you want a second MX record that redirects to a new imscp server, the only problem I can see is to replicate the data of your primary imscp to the second one. Since we're talking about two MX, we are talking about the same mail domain too.
In this case you can try different cluster/mirror/sync configurations. It depends on your server configuration/operating system. You need to remember that the second imscp has to be sync in realtime, especially if your MX will have the same PREF value. Your two networks can do a realtime replica without loose performance and create malfunctions on other systems?

If your two networks are behing the same firewall with just one adsl/line and you just want a HA solution, you could use a different approach to this problem using a reverse proxy/load balancer (such HAProxy), but still you need first to create a solid replication system beetwen the two servers.

Nothing is worst to build a perfect failover solution and your failover server is not updated or worst corruped.

I'm working on a simple (maybe not elegant) replica solution for my imscp (on Ubuntu 12.04), so if you want to talk about it I'm here.
If I have misunderstood your request and wrote a buch of crap, sorry

AS Secondary DNS, i would recommend Hurricane Electric. If you don't have huge zone s (Exceeding 10000 records) they are perfect and FREE as in Beer ;).

https://dns.he.net.

Just configure bind to accept AXFR from HE and make some changes in the templates and you are up and running.