Most split keyboards these days have a microcontroller in each half. IO expanders like the MCP23017/MCP23018 are fairly expensive, quite slow when used with microcontrollers like the ever-popular Pro Micro's ATmega32u4, and they really limit your hardware options on the secondary side.

It usually makes more sense to just stick a Pro Micro on each side, and have them talk serial over TRRS. As a bonus, you can connect it to your PC from either half.