I have a Plesk installation using a single IP with NS1 and NS2. However, I hired an additional IP and I would like to set it for NS2. I was wondering how can I configure the same, so that IP 1 is NS1 and IP2 is NS2.
If I understood the question correctly, you need to update DNS template/zones and set new IP address as a value for existing NS2 record in Plesk: DNS Template. Are you going to use an additional server with that IP-address as a secondary DNS server, aren't you?
It's rather a pointless idea to use two IPs on the same server. If it's required by the registrar, oh well.
Otherwise, you only protect against the very specific scenario that one IP fails. Give that it's the same provider, and I'm guessing same subnet, this is really highly improbable. I'd recommend just using your registrars NS.