The Dayton DSP-408 has two ADAU1701's to make it a true 4x8 DSP. How would I be able to do this on my own? I have the Sure APM2 that has a ADAU1701. Is it somehow possible to transport audio between two APM2's to increase the in/out count? Thanks!
You just have to connect the two serial ports together so they will send audio data to each other. One will be the clock master and the other a slave. You can use two I2S ports, but that uses up more GPIO pins, or you can use TDM-4 to send four channels between the parts. This would limit you a little or you can use a TDM-8 format and be able to fully share all channels between the two DSPs. I usually try to use the slowest clock speeds as possible to avoid issues with signal integrity and timing. As long as they are close together you can make TDM-8 work. I suggest running the clocks using a ribbon cable with ground between each clock on the ribbon. So for BCLK, LRCLK and SDATA lines in both directions you would need an 8-way ribbon.
You also will need to use the same master clock for both boards. So this would be yet another signal to jumper between the boards.
As you know, where I work we made a 12-in mixer with no less than six -1701's arranged this way, so what you suggest will definitely work. The DSPs are interconnected via TDM-8 with "on-ramps" and "off ramps" within the individual DSPs, in an arrangement we nicknamed the "Capital Beltway." In case anyone needs to know -- there's a three-sample latency in TDM transfers from one -1701 to another, Also we found it necessary to include a small damping resistor in series with the MCLK source.