I can't find any S/PDIF mirror mains button in UniversalControl for MacOs. (PreSonus Studio 68c)
UPDATE: (General system setting): I found it under either MacOS sound settings, and tab "setup speaker". When choosing speaker S/PDIF occur in drop menu on any speaker.
And another example I tried with Logic Pro, Preferences, Tab "I/O assignments" needs to be change to speaker Outputs 5-6, and eventually check the box "mirroring". It may be the same way for other software that control other studio/soundcard directly.