Several different ways to do this kind of patch bay. Easy way, use a busy line to know which pic is going to write/read the I/O expander (74hcxxx)...

each PIC interaction will begin AND end with an open collector output.. well says they will be set as input.

I'd already done a 948 studio audio patch bay controlled by 48 different PIC like that in the past, i used I2C i/o expander for that. Still on the market, no problem at all.