Connecting two microcontrollers - project - repost

I have to do the project with connecting two AT89C51RC2 microcontrollers. The inputs of the first microcontroller have to be mapped on the outputs of the second one, and also the inputs of the second one have to be mapped on the outputs of the first one. You have to use RS485 communication and MODBUS protocol for connecting this microcontrollers. This project must be done into C language, Keil Microvision. The task seems to be relatively easy, but I have no idea how to start. Can anyone help me? Thanks!