Two PCBs, PCB interconnects, two separate housings, the mechanical design of keeping the two pieces connected when they should be connected...and yeah. At some point, I need to talk to the Workman and Colemak communities about the ideal mappings. For now, I'm running with:
https://github.com/obra/KeyboardioFirmware/blob/master/layouts/workman and
https://github.com/obra/KeyboardioFirmware/blob/master/layouts/colemak