samreen_kiran, there is little use to use the Modbus protocol if you only want two Arduinos to communicate with each other.
If you only wish two Arduinos to communicate at short distance, then just use their TTL signals.
If on the other hand you need long distance, say greater than a few metres, then read and learn about Nick Gammon's article on using RS-485 to connect two Arduinos, mentioned in the very first post of this thread.
Also, look in the playground for other examples of methods that might suit your needs better.
Paul