If you use one pin you have to keep switching them from input to output, which you can easily do in software - normally both PIC's would be set as inputs, to send data one would switch to output, send the data, then switch back to an input and wait for the reply. The second PIC, havng received the data, would switch to be an output, and send the reply back to the first, then return to being an input.
You need to decide exactly what you want to do, and how you want to do it.