Hello srik;
Basically, to establish communications between PLCs, you have to decide on protocols, topology and data exhange parameters. Modbus is a very well supported protocol in the PLC world, and is available on most (if not all) PLC platforms. It is also relatively easy to setup. So there is no absolute need for both PLCs to be of same make and model as long as both support the same protocol(although programming issues will be much simplified if they are). One of the stations must be setup as Modbus master, the other as Modbus slave; addressing and connectivity (RS-485, 2-wire, at 9600 Baud, Even parity... for example) must be consistent on all stations of the network.
Note that there are other industrial protocola on which you can do the same: Profibus, Ethernet (make sure it is the same version on both PLCs)....
Then comes the hard part: setting up the data to be exchanged. Amount and type of data must be decided on, and exchange tables must be structured (data areas reserved for communications). Say, if you have a Start bit that you want to send to the auxilliary PLC, it is better to set it in the Input Exchange table, use that Input bit to command the process you need to start (in the auxilliary PLC program), and send confirmation back to the master PLC through the Ouput exchange table, for example.
Come back to us if and when you need to clarify things.
Hope this helps,
Daniel Chartier