It is important to understand the aspects of mind that they do share. They share some motor control, some common sensations, and probably share some aspects of imagination — that is, the ability to reconstruct sensory images (visual, auditory, olfactory, etc.).
This sharing of some aspects of the mind, but not others, is remarkably consistent with classical Thomistic dualism. In Thomistic dualism, the human soul is the composite of three powers: vegetative, sensory, and rational. Vegetative powers are what we today call autonomic physiological control — control of heart rate, control of blood pressure, control of growth, reproduction, respiration, hormonal control, etc. These are unconscious powers that make life possible in the most fundamental way.
In light of this Thomistic understanding of the soul, the abilities that Tatiana and Krista share are the material powers of the brain, which we expect them to share, because they share brain matter. What they don’t appear to share is the immaterial aspect of the soul — reasoning in an abstract sense, and personal identity, individuality, etc. They are separate souls who share some material brain tissue, and thus share some material powers of the mind. They do not share immaterial powers of the mind, because immaterial powers can’t be shared, because immaterial powers aren’t material things that can be common to two people. More.
Oh, but wait! Isn’t consciousness an illusion anyway? There, that’s good for another few rounds of tenure.
See also: 11-year-old conjoined twins have a connected brain, see through each others’ eyes, but have separate minds Sobering thought: Had the girls been aborted or left to die, we would never know what they can help us understand about how the human mind really works. We’d be stuck with the elegant essays fronting naturalism. Much to be preferred in many quarters.
The difference between science and reductionism Chaberek: Reductionism begins not when scientists speak about material and efficient causes alone, but when they (or anyone else) claim that scientific knowledge is the only possible type of knowledge, or that science explains everything, including the mystery of life.
Nature, as defined today, cannot be all there is. Science demonstrates that.
Post-modern science: The illusion of consciousness sees through itself