There can be different approaches:
1. Use Modbus TCP devices that support more than one connections in parallel. You run 2 similar instances of Rapid SCADA, which receive the same data. Both the instances are active simultaneously.
2. You can use the new module Rapid Gate which pass data from one Rapid SCADA instance to another in real time.
But when the primary server stops, you have to somehow switch device polling to Communicator on another server. For example, stop the 1st Communicator service and start the 2nd Communicator service. May be it is possible to do it automatically by some condition.
The important thing is a scheme of connection with devices.