So if you see the UDIO-8 as an output option in MC, you should be able to go into DSP studio and set output channels to "stereo in a 7.1 container". That will give you a stereo mixing target (i.e. JRiver will downmix multichannel to stereo) but will give you six additional blank channels. Then in parametric EQ, you want to choose the "mix channels" filter option. In the options on the bottom, choose "copy" and copy the left channel to another available channel (like SL). Then make another filter and copy the "right" channel" to another channel (like SR). Once those filters are enabled, if you start playing audio, you should have two left channels and two right channels coming out of the UDIO.
You'll still need to figure out which channels map to which physical outputs on the UDIO (i.e. is SL the fifth output or the seventh?), but that's something we can't answer and that you should be able to figure out with a few minutes experiment plugging and unplugging.
When you're ready to do active speakers or multichannel, the same basic techniques will work. Just choose an output format based on your mixing target and the number of channels your device has available. For example, if you want a 5.1 system where the mains will be actively bi-amped, you'd choose "5.1 in a 7.1 channel container" and then use the empty RL and RR channels as the extra outputs for your bi-amped mains.
According to the Manual - https://www.minidsp.com/images/documents/U-DIO8%20User%20Manual.pdf - you can only connect the UDIO-8 to a single DAC!
It might be possible to do more than one DAC given that SPDIF/EBU outputs bring their own timing information with them (unlike, say, usb outputs). Only one way to find out!