Aniroodh Sivaraman, 

It may be based on intellectual attachment, where two entities worked together in something having to do with science or mechanics, where they work to develop inventions.

They stay close together and become attached to the success they achieve and the popularity and fame they achieve. Then by that force, they remain in association hereafter and when it is time for the next body, they again are together and so their reproductive bodies fuse together early on in the development of embryos.

Nature only facilitates the attachment and is not responsible for it in their cases. The separation by the medical personnel does not remove the attachment and the twins will have to work that out because if they do not, they will again come back as conjoint during the next birth development.

Right now, even though they are separated that attachment is still active. Surya filled some information regarding that when he said they had high blood pressure when they were separated initially. This shows that nature can be very responsible to our psychic needs and conceptions. We must be careful so as not the use the mind whimsically because nature may manifest what silly or grand event we are conceiving.