Do you mean to say that you want to mark your IPAddress of your SERVERB to 192.168.120.1 and SERVERA to 192.168.120.2?
How are the applications connecting to your database. Are they using IPAddress or ServerName/InstanceName?
Once you change the servername all the instances running on the new server will change to reflect the new name.
If you are just swapping the IPAddress, application configuration files using IP address to connect to your database instances should be changed appropriately.
Prasad Bhogadi
www.inforaise.com