In vim, when you have the same file open in different windows any changes immediately appear in the other window. They are the same buffer, there are just two windows to it.

You can see this by opening a file, and hitting "<Ctrl-W> <Ctrl-S>" to split the window. Do some editing and you'll see it in both places. ":ls" will show only one buffer. And if you have display of the buffer state visible, when you ":w" all the windows will update to say they were saved.