Imran,

If you want to acheive communication between two MCS-51 controllers (assuming all of them support traditional multiporcessor communication mode on the serial port) its is quite easy, but you have to use the serial port on the controller. Checkout the MCS-51 hardware manual, serial port section for details on this mode. You can program the serial ports to work as a shift register and tramsfer data at high speeds between contollers.

The other method is to use a parallel port, namely P1 and use a few other signals for handshaking. Maybe you could use a P3 port line to interrupt the other controller and ask it to read its P1 port. This is, P1-P1 transfer using INT0/1 lines either way.

Next, you can use two of the spare port lines (one for data SDA and other for clock SCL) and write a I2C driver to read and write bytes. This is a software approach and is very slow. Some MCS-51 variants have inbuilt hardware I2C ports, but I assume you are not using those variants.

Hope this helps.

girish