how to record video with mixture of line-in and out?
Install Soundflower
Open QuickTime Player and choose File > Movie Recording
Change the input device to Soundflower (2ch)


Create a new project in Garageband
Set the output device to Soundflower (2ch) in the preferences


Add one audio track for the background audio and another for recording the guitar
Enable playthru for both tracks


Start recording in QuickTime Player and Garageband