Conjoined twins need to be seperated because one has cancer and they both cannot survive the treatment. The twins become a mirror/metaphor for showing Sean and Christian how their relationship works and they, too, cannot be separated.
This episode was a great example of Ryan's gift of writing. Taking the tangible: the Rosenburg twins and their struggle for life and love of one another and mirroring the intagible - Sean and Christian's love for each and their relationship. No matter how imperfect a relatioship is, it survives for a reason. Each party invovled must accept that or it will fall apart and each will die. One cannot live without the other. Excellent episode.