Alright, I'll try that. Thanks to both of you :3
Nope, it's not working. I made two simple test masks and left the parts where the first mask is colored black/transparent on the second mask, and the parts where the second mask is colored black/transparent on the first mask. The first mask shows up fine, but when I want to add the textures of the second mask, they always overlap with the first ones, and then everything just gets brighter and brighter with each new texture. I saved them as test1Mask.png and test2Mask.png.
What did I do wrong? :/