I'm starting with Adrien Kohlbecker's 65816 breakout board.

I am aiming at parallel processing, using DMA channels based on address decoding, so that each CPU sees its even numbered banks as the opposite CPU's odd numbered banks.

I have also implemented a programmable interrupt controller that uses the 65816 VPB line to perform interrupt vector offset. Since each DMA channel can have its own address space of devices, the programmable interrupt controller is capable of not just interrupt prioritization, but of directing IRQ from any device to any CPU, under software control.

https://github.com/Aronnax9000/Vega816