Hearing only one channel if you upload to YouTube? Read on...
iShowU Instant recording two independent tracks of audio when both "app audio" and "mic" are enabled. It turns out YT plays only one of them. It's done this for years. Who knows why!
Here's some workarounds / fixes:
Upload using Instant
If you upload with iShowU Instant, it'll merge the audio tracks into one before uploading. Easy!
Compress to one track, when recording
Another option is to merge the audio tracks into a single track when recording. You can find this option under the Output tab, within the Advanced tab. This way, you can upload using anything you like, and you'll hear both track of audio.