Basic dharma theory tells me that it is definitely not possible to perceive both fingers simultaneously
That's kind of an advanced book.

There are no body parts within each of our own bodily experiece, only sensation.

Unless you are a natural you can't see how fast the mind works or see if awareness arises simultaneously in the beginning. It's all a blur until we have practiced long enough.

I guess your being taught to see things as they are in your own experience. While one does this kind of practice one sets concepts aside. Ideally even Dhamma theory could be put aside when one gets the hang of the practice.