I have set up my SPDIF in port as two mono channels to receive two independent mike inputs from my Focusrite Voice Master Pro (this unit is a single channel strip, but it has an external input to enable a mono signal to use the unused SPDIF channel of the A/D converter). It works perfectly, with a notable exception, discovered through a call to tech support yesterday. No direct monitoring is available when it is set up this way. To direct monitor, the SPDIF in has to be set up as a Stereo input. With that setup, and an actual mono signal coming in, the monitor signal is audible, but in only one ear.
Just so you know.
Fortunately, the Focusrite unit is itself capable of direct monitoring. Makes things a little strange using it in combination with the other MR816 inputs, but at least it’s working.