Help! Keying between textures in jit.gl.sketch

I have keying working on a single videoplane with 2 different textures, using apply decal on one of the textures. I'd like to get the same thing working within jit.gl.sketch so I can have control over the dimensions of the two textures independently. However, I can't get this to work for the life of me. I've attached an example of what works and what I think should work in sketch.
The crux of the problem is that if the movies are different aspect ratios or different from the Output window, I'd like to be able to keep them at their original aspect ratio. The videoplane method doesn't allow this.
Any Help Appreciated!

Pete