What is exactly Your task?
You can use second 80x51, simplest hardware variant is to
use 2 shif registers - one with serial_input/parallel_outputs and one with parallel_inputs/serial output.
Other choice is to use only par_in/ser_out registers - 3 chips with 8 inputs each == 24 keys. Wires will be VCC,GND,Load,Clock,
Data - 5 wires. With HC (74hc165) chips maybe You can combine VCC and Load signal into one wire.
regards