Part-time MSc while undertaking PhD Study?

What If I already hold the required knowledge regarding the additional MSc and just want to pick up the qualification for the sake of appearances to bolster my prospects as a post-doc? Of course this would fall second to any publications I have as well as the PhD itself.

Both very valid points and I guess that it is best I focus on my current research. I just planned to transition from numerical mathematics for engineering/classical physics to mathematics for astrophysics/cosmology as a post-doc and I felt the qualification would have helped since my previous formal education has not covered this. Of course I have self taught a lot, and have enough time to learn the relevant subject matter but it may not be viewed as equivalent in the eyes of the faculty.

I wouldn't say getting an additional MSc would necessarily validate what you have learned when applying for a post-doc position. Even if someone with a traditional MSc + PhD applies for a post-doc position, they may have forgotten most of the intricacies of their MSc curriculum.

For the most part, PIs who hire post-docs with a background outside of the PIs own research specialty will understand that a post-doc may only have conceptual, and not working knowledge of the field. However, being a post-doc, you will be expected to be able to catch up quickly.