The second proof begins with bodily parts being united or separable. If their union is not essential in such a way as to prevent all separation, their separation is described as possible and therefore within divine power.
If each separated part could be split again, the proof seeks a final indivisible terminus and calls it an atom.
The real dispute is not over divine power as such. It is over whether the successive acts of separation posited by the proof are intelligible possibilities and whether their possibility entails a terminal part.