Some years ago I did a project which used two computers and three PLCs.
The communication protocol was MODBUS, and the electrical connection was RS485.
The two computers were MODBUS Masters. In that system only one computer could be Master at any one time.
The two computers were connected in a dual redundant arrangement where one computer as "online" controlling the process, and the other was "standby" in case of failure of the online computer.
Whichever of the two computers was online, that computer communicated with the PLCs.
The three PLCs each had a unique MODBUS address. Addresses 1, 2, and 3 if I remember correctly.
JimB