Is ServerA the primary name server in your cluster? When you add to ServerA, it pushes to the DNS cluster (NS2/Slaves)?
And on ServerB you aren't running local DNS, it receives the update from the primary?
If I'm following you correctly, you'll want to run ServerB as a multi-server and check the options to share your Zones (doing this from ServerA, which would now be DirectAdmin server, to ServerB DirectAdmin). You don't have to use ServerB to do anything else if you just want it to do backup DNS.
Install custombuild, set your messages to go to a good email, set your messages / events to auto delete after 30 days, make sure you secure ServerB if you won't be logging in for a while (SSH port, DA port change, auto updates enabled / custombuild updates enabled), and maybe rkhunter or something to help protect it auto-security... might even help to run Clam type service even if you don't plan on putting a lot of sites on there.
Even as backup DNS it could be at risky for DDoS and then ServerB hack where they could edit/spoof DNS and redirect clients to fake sites.... Not that any of that's going to happen! If anything, you're more secure on DirectAdmin because it's using all the real packages, not some random RPM with "-cpanel" attached to it that you have to wait graciously for the WHM gods to update, while the rest of the world gets real time updates.
Hope this helps. I know a great deal about cPanel, if you can explain in more detail.. I'm doing live migrations cPanel -> DA as I type this. So far so good, I haven't found anything DA can't do that I was doing on cPanel/WHM thus far with these new changes (Icons Grid theme, cpmove restore, etc.).
