Hi, I am wanting to record my USB microphone’s audio alongside friend’s audio in a Skype call in Audacity. Is there any way I can record both at once?
When I select “Microphone (Blue Snowball)” it only picks up my voice, then when I select “stereo mix” it only picks up the Skype call…
I would be so unbelievably grateful if you could help me with this.
Thanks in advance, Rhodes 
You can use Skype Call Recorder.
http://atdot.ch/scr/
or Pamela Business or Pamela Professional.
http://www.pamela.biz/en/products/
Pamela is the full pro application and will give you two independent sound tracks so you can edit and filter one voice without interfering with the other.
Koz
https://forum.audacityteam.org/t/help-recording-with-a-mic-laptop-and-headphones/35068/1
You are warned about trying to do production in MP3. MP3 compression sound damage builds as you edit. Also, some packages will not give you a split show which can be OK if the sound quality and volume work OK in one pass.
Some people do manage to get Audacity to work recording a full podcast show with Skype, but the process is magic and unreliable.
Koz