Can I simultaneously record video from a MacBook webcam and the iPhone screen?
You can open a second instance of QuickTime using
$ open -n -a QuickTime\ Player
#       │  └ Specifies the argument is an application to open rather than file.
#       └ Open a new instance of the application even if one is already running.

Each instance can start a New Movie Recording with different inputs.
You can then use iMovie to combine the two videos, using Picture in Picture:
Create a picture-in-picture effect in iMovie on Mac