I just did this recently and for one device it’s simple. using the MR Editor you have your outputs on the right side. normally you set the mix on 1&2, so go to 3&4 and setup the same mix.
Now the only trouble comes where you want wdm to pass through multiple outputs. I’m not sure if you can do this with one mr816, but you can with two. Connect the adat cables between them. Set them both to adat mode. then set wdm to use adat 7&8 of device 2 (you could use spdif if you want, but I don’t.)
make sure you mute the adat 7&8 input on the card you assign the wdm to to remove the feedback loop.
there you have it, now wdm audio comes through on adat 7&8 of the first device.
Another option, and I also do this on occasion, is to have another sound card in the system, use that for windows sounds and bring in the output to a couple of channels, or spdif. this is the simplest way by far. watch your system performance though, some systems don’t work well with multiple devices installed.