Separate Audio from Video does not work

Dear all

I shot an interview with a two camera setting (SonyA7III):

Cam1: Recording to Atomos Ninja V, Audio from Sennheiser XSW

Cam2: On a slider, recording to Sony SD Card, without specific audio

After importing the clips to Adobe Rush, I am only able to separate the audio from the SD card recordings but not from the Atomos recording. The UNLINK option just does not show up. 

What am I missing here?

Looking forward to your inputs.