Gugaplexing 24 LEDs Using 4 Pins

I want to create a digital TicTacToe game using LEDs, pushbuttons, and a Digispark ATTiny85. Pins are a premium on the Sparky85's. At a minimum, I'd need 18 LEDs lit & 18 pushbuttons (9 for X and 9 for O). Shift registers would only use 3 pins and seemed like a suitable solution for either the LEDs or pushbuttons. I haven't seen anywhere that 2 different 74HC595 shift registers (1 for input & 1 for output) are controlled by the same SER (Data) & SRCLK (Clock) pins but with different RCLK (Latch) pins. The Sparky85 only has 1 MOSI (for SER) pin and 1 SCK (for SRCLK). I'm not a coding pro, but it seems like a challenge to use the same data pin for 2 different registers. Guess I need to find some more options.

After a search for different solutions, I stumbled upon charlieplexing. Very interesting concept and new to me, but I'd need 5 pins for either LEDs or pushbuttons. Then, 1 single Instructable by cedtlab mentioned gugaplexing (GuGaplexed LED Heart)...and that was back in 2008. Still a lot of good info on the difference between charlieplexing and gugaplexing. I did a lot of searching around for more information on gugaplexing, but most posts/articles reference the EDN Article from 2008 (also attached original article photocopied). Unfortunately, they only show the logic table for 2 pins (or 4 LEDs). There was the 24-LED circuit so that helped some. Time to apply some critical thinking skills.

The total combinations possible for N pins with 3 different states is N^3 which gives us 2^3=8. The formula for available outputs is 2*N*(N-1) which only gives us 4 outputs for 2 pins. That meant there are some invalid states for all possible combinations, but I couldn't find anything to tell me what those states were. The table for the 2-pin gugaplexed circuit didn't have 1 pin high/low and 1 pin at Z (high impedance). My conclusion was that you need 2 pins in an output state (1/0 or high/low) and anything connected to a pin at Z (input state) would not work. So what does that look like???

The results of my critical thinking skills and horrible drawing abilities. Attached are the schematic and logic table for 24 LEDs. I don't need P3 to P4 or PR3 to P4 because I only need 20 lights, but I put them in there in case they help someone else out. Since I'm using RGB LEDs, I need 2 legs of the 4-legged LED to be biased in the same direction because they'll be common cathode or common anode. That's why the LED numbering is a little sporadic.

To get 1 LED to light, only 2 pins are in an active output state(high/low) and the other pins are in a high impedance input state. The same principle will work for 3x3, 4x4, 5x5, etc.

Well I hope this helps someone looking to X-plex the heck out of something!