I think both of those theories may be true. It almost seems to me as if Vincent, the 'twins'' father kind of knew what was going to happen from the beginning which changes my estimation of him DRASTICALLY.
Im starting to wonder if "the twin" had something to do with Ciel being kidnapped and sacrificed/killed to begin with. Or if Vincent knew this was going to happen to see who was the "strongest" to take over the Phantomhive family. Literally at this rate I think Toboso-sensei is trying to throw anything at us just to see how we react (●￣(ｴ)￣●)...but even with the new "drama" this is still one of my favorite mangas
Ive read books where siblings/twins in particular are used as "backups" or "shadows" for their sibling. Either one is the heir and being protected, or the other sibling is sick. If I remember correctly Shinsi Doumei Cross follows something similar. (except totally different story line) But I think it was something that was practiced in history, especially in royal or prosperous houses...