[B]Communication between PLCs[/B]

you should use same communication protocol (profibus , modbus etc.)

Yes I'll use same communication protocol. But how will I send the data from local PLC to partner PLC or vice versa ?

i dont know your other plc brand but for siemens you can use SFC14 and SFC15 read and write to profibus .

Choose a common protocol that they both talk and use that one.
If Ethernet is an option, socket interface could be possible but it's not easy to implement.
Usually there are protocol converters that can be used... and manufacturers also make hardware to allow the use of protocols from other brands.
Allen Bradley, for example, has a Profibus communication card for their ControlLogix

One option is to use a computer to read from one controllers protocol and then write the information back into the other controller using a different protocol.
This can be accomplished using AdvanceHMI.
https://sourceforge.net/projects/advancedhmi/
The protocols supported are:
Allen Bradley DF1 RS232 Driver
Allen Bradley Ethernet/IP Driver for SLC,MicroLogix, PLC5, ControlLogix, and CompactLogix
Beckhof TwinCAT Driver, ModbusTCP and ModbusRTU drivers
Omron Drivers
Drivers are accessible via VB or C# code

Just a thought,
Regards,