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.

UART

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

keyboard.split_type = "UART"
keyboard.uart_pin = board.SCL

Config

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 "target" half

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_target = "Left"
OR
split_target = "Right"

and then in your keymap, add the line

from kmk_side import split_target