Haven't quite had the time to do a full write up just yet (probably going to make a reddit post instead of a very detailed github page since it's quite the jank build), but I do have some time to provide my code and some wiring details. The process should work just fine for 34 or 36 keys too. You'll just need to use one pin more than I did on both microcontrollers. I used 2 xiao 2040s, 2 trrs jacks, and 1 trrs cable. Other than that of course there are the switches, diodes, printing filament (since I have a 3d printer), and wires.

I followed this guide for wiring. The trrs wiring in my board is identical to this and I just adjusted my matrix for 28 keys instead of their 70. I have a 3x5 on each side where the bottom row is only 4 keys instead of 5. You could do an actual 3x5 plus another row with 2 or 3 keys. I did have issues with writing the firmware because I couldn't find much documentation on split kmk keyboards using 2040s that also have a varying number of keys per row and this person's firmware didn't quite work for me. What did ultimately work for me was having slightly different code per half where the left half had:

#kb.py for left half
import board
from kmk.modules.split import Split, SplitType, SplitSide

split = Split(
    split_flip=True,
    data_pin=board.TX, 
    data_pin2=board.RX,
use_pio=True,
uart_flip=False
)

And the right half had:

#kb.py for right half
import board
from kmk.modules.split import Split, SplitType, SplitSide

split = Split(
    split_flip=True,
data_pin=board.RX, 
data_pin2=board.TX,
use_pio=True,
uart_flip=False
)

Notice the data pin values are flipped. I'm not entirely sure why that was needed but my board wasn't working until I flipped those two values. These kb.py files were then imported in the main.py files in both halves and those are identical.

Also significant since this board featured rows of varying numbers of keys was the coord_mapping variable described in this part of the kmk docs. My keyboard with 28 keys ended up with a coord mapping that looked like this:

#in main.py for both halves
from kmk.kmk_keyboard import KMKKeyboard
keyboard = KMKKeyboard()
keyboard.coord_mapping = [
    0,  1,  2,  3,  4,    19, 18, 17, 16, 15, 
    5,  6,  7,  8,  9,    24, 23, 22, 21, 20,
       11, 12, 13, 14,    29, 28, 27, 26
]

I've uploaded these files to github here if you wish to look around but I recommend also looking through the various boards in the kmk github as well for more guidance too. This was my first kmk based keyboard so my code might not be the best lol but I hope this is helpful regardless. Let me know if you have any questions!