Erasing Image, show underlying

[MentalGear]

Use-Case:
Users should be able to upload various images, erase part of images, and stack them over each other (underlying image visible through "erased area") for some kind of a collage.

Question:
Given the limited amount of layers we can use, I was wondering if this can be done without layers?

[MentalGear]

I was able to get it working up to the point that a masked part of the image (yoda) is visible above the below image (darth). Yet, I'm not able to display the whole stage anymore even after trying most composition options. Maybe someone could give me a hint how to proceed, or if an "eraser tool" to show the below image is even possible ?

Code:
https://jsbin.com/dapevuniza/1/edit?html,output