This requires some planning out.
Once I have mounted the Pi's to the pedalboard I need to measure the space left for the enclosure:
This will determine the amount of buttons and functions I can fit into the box.
Reaper Pi - essential controls:
Channel selection (two buttons? Or an encoder?)
Arm track to record (button)
Record / Stop (button)
Desirable: mute/solo/undo buttons. Could be added as held button press features.
Pisound - essential controls:
Pedalboard up/down (two buttons)
3-4 effect switches (with LED feedback to show on/off)
Analog inputs broken out to trs jack's for adding expression pedals.
I2c breaking out for possible extension button boxes to be added.
Research to be done:
Gut instinct is to run two Arduino Pro Micros as co-processors for this to give two native USB ports out. One used as a usb midi input for the Pisound, and the other as a usb midi device for shortcuts in Reaper.
I've picked up a Raspberry Pi Pico recently, and running Circuitpython on that also allows for native USB use. They're really cheap, so may also make for good choices for this.
Other options: run a software serial port out to a hardware midi jack and send midi data to the Pisound via the midi jack, dropping the need for dual controllers.
If there's going to be a network established between the two Raspberry Pi boards there's a shout for using an ESP8266/ESP32 board to send wireless MIDI, perhaps.
There's even a shout to use a Pi Zero in USB ethernet gadget mode to share one of the Pi's network connection.