If the other speaker's voice has not been recorded, unfortunately there's no way to retrieve it afterwards. In theory that sound could have been recorded at a seperate track, but that's most unlikely if you do not have a clue on how to do that.
If you do want to check: open the recording in i.e. VLC Media Player and have a look at Audio, Audio Track:
If you want to next time have both voices heard, please provide more information on your current OBS Studio setup. How do you get your own voice in (i.e. a microphone at Audio Input Capture source) and the other speaker's voice as well.