OcularCash
My other hack to get at another texture for an effect was to make the sprite 2x wide, put the second texture into the right half of the sprite and then have the effect combine the textures as needed in the left half and render the right half as transparent, regardless of its content. I _think_ this has similar performance as rendering one sprite, then overlaying with another and rendering again as Ashley suggests (which we do for the normal map effects now.) I know there's also the possibility of discarding the fragment for the right half, but I have seen mixed comments on how performant that is. It also is less dynamic, in terms of your need to pre-bake the other texture in and you would need to do that for each frame of animation.
The other hack I used, was putting a second texture depth data into the alpha channel, but that only works if the actual sprite does not need alpha, and also I have a sneaking suspicion it has some other side effects, even if the shader zeros out the alpha on the output color. It's also a pain to get a 'clean' insertion of data only into the alpha channel with Gimp, Photoshop, etc. without affecting the RGB components.
I've also made the request to add more texture sources for effects/sprites, but so far no response.
This is the effect that used the 2x sprite hack:
construct.net/en/forum/construct-3/plugin-sdk-10/parallax-map-released-effect-141155