Here's a picture of two interconnected V1 units. The new version has smaller 1.5u thumb keys, and uses a rectangular PCB.
The main issue with the old design was that the Teensy required the back to be raised off the table slightly more than I would like.
Using an on-board micro allows the PCB to be lower, although there's still a gap required for the I2C and USB connections to be underneath. I may do another version with top mounted connectors to allow the PCB to lay flat.
I'm also doing a Bluetooth adaptor which should allow a BLE Keyboard connection for both units via a Bluefruit BLE Feather M0 module, and may provide a couple of additional thumb buttons.