How to do picture in picture with Premier Pro Multicamera
You can't select, say, two video tracks from your Multi-cam sequence and show them at the same time. You're going to have to go at it separately. A Multi-Cam sequence is really for going back and forth between angles, not to show two or more angles at the same time.

A solution:
- On Video track 1, put your base image or shot.
- One Video track 2, put the image or film you'd like to have appear over the base image. Sync the two video tracks together, by hand. That is, don't create a Multi-Cam sequence: merely put them one on top of the other so that they start at the same time of the take. 
- Then, use the "Crop" effect and the "Scale" tool to reduce the size of your Track2 image and move it in the appropriate corner of your base image. 

Voila! You have two angles of the same event playing at the same time in the same overall image.