Hallo Ertan,

you could porceed with the following schematic, if you insist on discretes: (There is some text below the schematic. I write this, because previewing showed me some gap...)


As you can see, 4 signal lines are necessary instead of 2, as you wished. It's possible to decrease number of signal lines further, but at the cost of drastical increase of number of chips.

To the circuit:
Bidirectional data line is achieved by the help of open collector circuitry. That's why you see 2 transistors. Interfacing is done by Schmitt-trigger inverters. RC-filters at inputs help to struggle with crosscoupling and interference. RC-filters at outputs of HC14 help to decrease rise time, also in order to prevent crosscoupling between the lines.
Keep in mind that there is no protection circuitry in the simplified schematic. But inputs are automatically protected by RC-filters and built-in diodes and HC14 outputs are also very robust. If additional protection is needed, e.g. for ESD, then at boths ends of cable fast switching diodes should be connected from each line to 0V and Vcc. Also the use of cable with common shield is highly recommended. Keep in mind, that shield must be connected at both ends to signal ground.

How does the circuit works?
Assume you will output signal for the LEDs. Then you give data on DATA line, and after stabilizing you let toggle CLOCK OUT from low to high. With the positive edge data at DATA line is taken by shift register CD4015. Repeat this still 2 times and all 3 LED information is transmitted.

When you will input keypad information, you first must load parallel data at inputs of CD4014, which represents condition of the 12 keys, into shift registers. This is done by applying high state at STROBE line. After stabilizing let toggle CLOCK IN line from 0 to 1. Afterwards remove logical high from STROBE. Again after stabilizing signa,l let CLOCK IN line toggling for pulling all information out of the two CD4014.
Finally, let toggle CLOCK IN line once more for pulling low state at input of driving transistor (located at CD4014). This results in switching-off this transistor and prepares DATA line for the sending of LED information. Of course, in the recieving mode, means when pulling data from shift registers CD4014, transistor at mcu must rest switched-off.
I think you will become very quickly familiar with the circuit.

Please keep in mind that this circuit is quick shot solution. It was drawn in half an hour, just developed for you in a free hour. May be that there is some bug.

Hope it works,
Kai