In the mean time you could use a splitter and split the mono signal to the left and right channels.
My source video comes from the output of a recorder which is hd sdi and i record channel 1 with commentary and channel 2 without commentary but wish to stream commentary on both channels to the livestream. Because the audio is embedded and in hd i cannot split it that way.Can you suggest a other way i am not aware of ?
Not exactly an inexpensive workaround... but you could use the SDI to Analog converter to get the separate audio... bring it to a small mixer..mix it as you need... then connect it back in to the external audio input for the ATEM.
EDIT: And... How are you livestreaminging using the H.264 encoder? Does the livestreaming encoder software have any audio mixing capability?