I beg to differ. The ambiguity is neither subtle nor is it to be tolerated.
First, comparing この傀儡の殻を永遠にわかつことがない and この傀儡の殻は二人を永遠に分つことがない, we notice that わかつ is used in two different senses in the two. The former, having この傀儡の殻 as the object, uses わかつ in sense of sharing, imparting or something like that ("Tear the body apart", your English translation, wanabee, can mean "break the body into pieces" but this reading is impossible for the Japanese original). This reading is hard to endorse, in view of what the brother and the sister have been doing. They shared the body! The latter, having この傀儡の殻 as the subject, uses わかつ as "sever the relationship." This is a more satisfactory understanding but I have mentioned above that this is syntactically a more difficult reading.
Second, you may encounter this type of sentences in live conversations and you still may be able to disambiguate them by contextual information. The topic of this thread is different. We are dealing with an artificial material where less context is available than in face-to-face communication. The authors should have take enough time editing out ambiguities.