what information should i know to do the following:
we will have 2 servers, each one will have a cPanel WHM installed and 2 will have DNS, we want to insure that redundancy is achieved on both servers, means the servers should work as active/standby unit. 2 cPanel active/standby 2 DNS active/standby
also does this have any impact on the mySQL database, do we have to buy another server to use it as the mySQL database or it's possible to sync all the cPanel information from one server to another.
What you are asking about is true HA/Clustering ....
This is something that is not directly supported by Cpanel but your could could setup an additional server to server as a HA/LB manager to serve as a reverse proxy to direct traffic to mirrored servers though keeping servers synchronized themselves may be a bit of an issue and would definitely need to be 3rd party as Cpanel only supports MySQL and DNS clustering --- not accounts and end user data.
You also MUST do the type of configuration I just said above to do this else you would have to deal with different IP addresses for each server exposed to the outside world and no quick or otherwise seamless way of switching without downtime and that would kind of defeat your point of doing any of that in the first place.
Otherwise, I would probably lean more to getting extra drives and setting up a RAID configuration so in the advent of a drive failure, you can switch over to the other drive(s) and this may be a lot more cost effective and less complicated to maintain than an HA/Cluster scenario and also don't need as many server machines either.
Another option would be to get a cloud server where hardware failures are very unlikely though worst case scenario, you could still in most cases deploy an exact duplicate copy of the server with all it's content usually within a matter of minutes up to an hour and get back online again very rapidly no matter what happens usually.
If you are looking to Cpanel for clustering in and of itself, your only real option is clustering the DNS and setting up your MySQL on a 3rd server that is called from both servers (or clustering the MySQL with manual setup) but synchronizing the account files and data would be completely up to you and will have to come from somewhere outside Cpanel.
The cost overhead of what you want to do though typically doesn't justify itself just to avoid what would usually be very short downtime anyway if any at all except for the most extreme catastrophic of circumstances which is why most would generally opt more for the RAID options.