Don't forget the ability to mix the hardware inputs back in with the playback on the headphone output.
Just so it's clear, my point is that the OP has a plan to achieve a goal... I'm assuming it's the best immediate option for him.
If you have a mixer and some cables, maybe a Y-cord, there's a bunch of other ways to do it too... but I figure the OP wouldn't be asking about using the spdif if there were other immediate options.
I don't have any knowledge of the little spdif boxes... so I just came by to read.
The first thing I imagined is that the OP is using the analog outputs for monitoring while overdubbing... so they are already in use and he needs another discrete send to trigger a hardware vocoder.
Duh!
It seems discouraging to see someone discourage someone from using the 2nd pair of outputs on the QUAD capture.
I thought maybe someone might be wanting to make a point of suggesting monitoring with a hardware mixer and using it to split the signals.
It might be an option.
best regards,
mike