kmk_firmware/docs/split_keyboards.md

Split Keyboards

Split keyboards are mostly the same as unsplit and very easy to adapt a keymap for. Currently UART is supported, though other modes will come later such as Bluetooth and i2c.

Useful config options:

keyboard.split_flip = True  # If your boards are identical but one is flipped, this option is for you 
keyboard.split_offsets = [6, 6, 6, 6]  # This is the how many keys are on each column on the "Master" half

Master Half

Choosing the master half can be done one of 2 ways. You can set this true or false if you only pleg in on one side.

keyboard.split_master_left = True

EE HANDS

If you want to plug in on either side, it can be done fairly easily but requires setup.

On each half of your keyboard make a file called kmk_side.py and add one of these lines to the file depending on where each piece is physically located.

split_side = "Left"
OR
split_side = "Right"

and then in your keymap, add the line

from kmk_side import split_side

UART

To enable uart it's as simple as adding this line, of course changing the pin

keyboard.split_type = "UART"
keyboard.uart = keyboard.init_uart(tx=board.SCL)