The OP asks whether the disciplines we call philosophy and physics (or science) arose from some more primitive philosophy that split in two. Such a position would make sense if the approach to the reality that we recognize today as science and philosophy could be found in an earlier philosophy that required splitting for them to exist separately.
A. N. Whitehead presents a different view of the rise of science. Science did not split from philosophy, but was a merging of two approaches of attention to detail and generalization that was always present but now had become a new mentality. This new mentality was "the most intimate change in outlook which the human race had yet encountered" (page 2).
This new mentality was more important than the science and technology itself: (page 3)
This new tinge to modern minds is a vehement and passionate interest in the relation of general principles to irreducible and stubborn facts. All the world over and at all times there have been practical men, absorbed in "irreducible and stubborn facts": all the world over and at all times there have been men of philosophic temperament who have been absorbed in the weaving of general principles. It is this union of passionate interest in the detailed facts with equal devotion to abstract generalization which forms the novelty in our present society. Previously it had appeared sporadically and as if by chance. This balance of mind has now become part of the tradition which infects cultivated thought.
The OP suggests:
So physics and philosophy were parts of philosophy but physics is not now part of philosophy.
Whitehead suggests as an alternative to consider is that the only reason we see science and technology as special today is because of a new mentality that did not exist before and which "infects" all of cultivated thought including both the individual sciences and philosophy.
Whitehead, A. N. (1929). Science and the modern world. Cambridge University Press. Retrieved on July 15, 2019, from Internet Archive at https://archive.org/details/b29978531/page/n5