At school, we didn't have any input in what depth studies are taught to us. The depth studies chosen are based on the teacher's interest, as well as the amount of resources he/she has on that particular depth studies. Since you are a distance ed student, I'm assuming the information are sent to you? In that case, if you feel that you would like to do 'Equality and Difference' but the school did not sent you materials regarding that depth studies, you could gather your own materials, as you are doing a PIP topic in that area anyway.
I haven't come across PIPs that are relating to conjoined twins. However, there is a consistent trend for those who did well in the past, are those who formulate their own theory and/or those who are able to apply an existent theory successfully to their defined areas of research.
Good luck with yours.