Setting up audio for screen recording and narration

Hi everyone

I feel really silly asking this question, but I've tried to figure it out on my own and failing miserably.

I have decided to try and use a USB microphone to record scren recordings and add narration.  In the past I have used a USB headset with an integrated microphone (Microsoft LX 3000) and I have successfully been able to record and hear the playback.

However now I am trying to use a seperate microphone and headset, storyline only seems to pick up the microphone and not the headset.  When I go into the screen recording setting, there is a singkle "source" option whihc is for the microphone.  I can't see an option to add my headset as a second source.  (not sure if this is making any sense).

I'm thinking there must be a setting somewhere that enables me to set up my microphone and headset as two source devices, but can't see it anywhere.  HELP..

Thanks heaps

Lorna