“Two Jans” is one of the “censored” episodes of Black Jack - it was serialized in 1974 in weekly “Champion” magazine but it wasn’t included in the original collective edition due to its dreariness. 

It features conjoined twins - Jan and Jan. They share one body - they appear as a two-headed boy with two faces and two overlapping brains. German researchers hired Black Jack to perform an impossible surgery - to separate Jans and - if possible - to make the other brain survive the operation.

Here we see Jans as they converse in their minds during sleep. As we learn later they promised themselves they will kill the other one who will end up as an artificially sustained brain…