You've stated this configuration works and I'll infer that means you've tested the fail-over onto the cloned server.r Which means I'm not sure exactly what question you're asking here; what you're unclear on here.
It seems you're planning to haul along both of your existing servers and your existing backup routine, probably one server at a time, and change your DNS and MX over as you have stated (possibly with both online with different MX priorities), then bring along the second server.
To make the DNS transition happen somewhat more quickly, shorten up the DNS TTLs leading up to the switch-over.
In short... If it works, keep doing it.
Aim your mail clients at both servers during the DNS transition, so you get the mail arriving during the transition.
I'd probably make a couple of extra copies of the servers, given servers can get dropped or stolen in transit.
I'd also consider performing the whole migration effort once, and transition to hosted mail. There are any number of entities around that will host your mail for you, or that will host slices or virtual machines for you that can run various mail servers. This unless there's a good reason to run local mail servers, obviously.
If you're envisioning running two mail servers running entirely in parallel and mirroring arriving mail across both servers in an equivalent of what's also called a multi-site disaster-tolerant configuration, that's probably more work and more cost than its worth. There are hacks to do that via IMAP and such, but I'd not want to have to clean up if something should go sideways with the OS X Server configuration. The simple approach is usually better...
