Based on what we know about the significant variability of one conjoined twin to feel a body part e. This argument generalises to other cases in surprising ways. While these implications are highly counterintuitive, it is dificult to articulate the relevant moral difference between these cases.
Conjoined twins occur in just one out of every 200,000 live births.
While the other could have far greater effects causing longterm mental and emotional damage. This was, after all, the antebellum American South.