That would be the ultimate extension of the practice indeed. To see the corollary in spirit, seemingly unknown in the tonal, as the "third element". One could even expand this awareness, and see yet the fourth and the fifth elements. Etc.
This one struck me at the time I grabbed it, though, because it reminded me of an experience I had, here in the mundane mental -- or in that place which overlaps the mental and the astral, but wishes to manifest in the physical.
On one of the occasions I experienced hypnosis, in the man's bed, he asked me to focus on a picture hanging above the headboard. I was looking at it upside down, which facilitated the eyes-rolling-back (which has a technical term I don't remember now.) It was a lithograph of two men dressed in butch cassidy sort of clothing. He told me to see the "third man" in the picture. I did. God knows what happened then -- it went to deeper trance at that point and I never could retrieve all the material in my attempts to do so.
But years later, before it was finally acknowledged on his part that he did all these things, I would look at that lithograph in his den and ask him, "So where is the third man?" Admittedly with a certain fascination for seeing his reaction. ~My own hypnotic suggestion to him, as it were.
Given that the retrieval of the memories never could cross the line of the "third man", though, I would have to say it's an effective technique.
I much prefer Florinda's and your extension of it, though.