Grease Pencil : Drawing on multiple layers at once

Okay so, when I draw/paint with GP i use a techniqe I call "color conjuring" where the idea is to mix colors on-canvas instead of choosing a specific color on the wheel, using Addition blending mode.

Now, in GP it looks like this: I set up pitch black viewport background. I make 3 GP layers, called R, G and B and then set up the blending mode to Addition for every one of them. Alike, I set the adjustments to their respective color and factor 1.

I then use the layers as colors, no matter what brush i use or color it will inherit the color of the layers and I start mixing by drawing and changing layers to get the desired colors.

However, to get greys you need equal amount of RGB and I thought it would be nice if one could paint on multiple layers at once, so it would be possible to duo, or trio the colors to be more efficient with this tecnique.

I guess you could Shift+LMB on the layers and it would highlight them and let you paint on all of them. Could also use the layer order for the Z order if that's an issue.