Communication between 2 PICs over 2 wires

I have 2 atmega644 processors that need to pass data 2 each other. Serial and SPI is not available on one of the processors. So I have 2 free pins on one processor and the other processor is largely unused. The logic flow is as follows 1. a 4 x 3 numeric keypad to be scanned for keypress on the first processor 2. Each keypress to be sent to the second processor (to be displayed on an LCD screen) 3. When a 7 digit string is available and the OK button is pressed it is sent to a decoding function on the first processor (already written) 4. The function returns a integer between 0 and 3 which should be passed to the second processor. Tasks assgined to each processor (P1 and P2) Processor 1 Scan and decode 4 x 3 keypad. Send 'start sequence' #FF to P2 Send keypad press to P2 (0 - 9 , del ) ('OK' button to be ignored untill 7digits string entered) Send 'end sequence' #FE to P2 Receive date string from P1 Decode the 7 digit string resulting in an integer (S) between 0-3 Pass '$S$' to P2 Timeout of 10 seconds between keypress to trigger 'end transaction ' #FD; Processor 2 Interupt to wait for #FF or #FD from P1 on input pin. if #FF then send incoming digits to display function wait for 'end sequence' from P2 and return a date string Wait for $S$ from P1 #FD used to reset sequence and return to 'wait' state. How this is solved is entirely up to you and this logic flw is only a guidline.