the obs output is virtual from your computer so u both u+viewers get the sound from realtek. on your desktop in the under right corner should be a speaker symbol, rightklick it and open playback devices. your realtek should be listed there, rightklick it and set as default. go to obs audio settings, in desktop audio device add your realtek, now it playback sound to your stream without any hardware connections. then plug your speakers/mixer or whatever on the realtk hardware output. done
i still dk whats the sense to split this ^^ but yes u can do this by useing a virtual audio device which is includet on your realtek device
again rightclick the speaker symbol and open record devices. u should have a device there called "stereomix" when its not there rightclick in the record window and click show deactivated devices. rightclick on stereomix and on activate. rightclick it again and select properties, click the second flag called listen.check the box "use as a playback device" in the line "use this as playback" choose your audigy device. then accept and ok, a last time rightclick on stereomix and set as default record device. accept and ok. go back to the playback devices window and now set the audigy as default playpack device, accept and ok.
now the audigy playback sound in 2 ways, to normal output and to the realtek which playbacks the same now and u got your audio splittet. when u want that the realtek is the default playback devive which feeds the audigy u have to use third partner software for the virtual audio device like stereomix. good software is voicemeter or virtual audiocable, with that u can choose 1 in to 8 out or 6 out to 2 in or whatever u want but u need to practice with the cables a while. its advanced stuff but really good....
gl with the mixing ^^
http://vb-audio.pagesperso-orange.fr/Voicemeeter/
regards