I prefer (and maybe many others) that the two buttons are separate from each other.
I use the Start8 button only for Start8 menu and nothing else. And I think that's how it should be.
Anything that the new Windows8.1 button does, should be left to the Windows8.1 button.
That's why I believe the two buttons should be separate from each other.
At least, Start8 should provide option to have two buttons, so everyone will be happy. Some will configure with one button (per your comment) and some will configure with two buttons.