How common is it for twins conjoined at the head to share thoughts and visual stimuli? I've always been interested in the possibility of a pair of conjoined twins, joined at the head, being able to read each other's thoughts, react to stimuli that one of them can't see/hear/feel, and being unable to keep secrets from one another.  Until recently, every pair of conjoined twins I learned about said that they are very much two different people with different personalities and emotions.  

Then I read a wikipedia article about Krista and Tatiana Hogan.  It was a newspaper article, unfortunately, rather than a piece in a scientific journal, but the description of the girls is exactly what I had been thinking of:

If one girl is blindfolded, she can see what her sister sees.  They share thoughts.  They can't keep secrets from one another.  If one of the girls hurts a limb that she controls, the other girl feels the pain as well as the girl who actually hurt herself.  

I'm curious as to whether this phenomenon has been observed before.  How common is it for conjoined twins to share thoughts, see through each other's eyes, hear through each other's ears, and be incapable of keeping secrets from one another?  Has it ever been observed before this pair of twins?
Short answer
Likely, the subject pool of craniopagus twins is too small to allow for scientific research. 

Background
As far as I can see after a cursory search, there is no scientific data available on the shared thoughts and experiences of craniopagus twins. Note that the incidence is only 1 in 10–20 per million births in the US. The rare occurrence, together wit the fact that neurosurgical advances increase the odds of successful separation (Browd et al., 2008) make them a truly rare phenomenon. On top of that, parents with conjoined twins will practically be living in the hospital with frequent, stressful visits to a variety of caregivers. In those difficult living situations likely the last thing they are waiting for is another white coat asking questions. In other words, the subject pool for research is minimized further by low response-rates when invitations for research participation are distributed (educated guess, being a parent).  

Reference
- Browd et al., J Neurosurgery: Pediatrics (2008); 1(1): 1-20