How to Cut a Double Page into half

Hi ladies..
there's a lady who asked how to divide the two pager to put into shutterfly online gallery.
So I decide to put it here, too. :)

Usually this is the way I divide page.

1. First open the page.
And use: Image >> Duplicate



2. Open Image >> Canvas Size



3. And then change the 1st red circle into half of the real size.
and change the 2nd red circle just like that. (move the grey square I put the red arrow to the left).



4. and you will get this:




5. Do the EXACTLY steps to the copied file, BUT move the arrow (on number 3 step) to the right instead.

I hope this is clear enough. :)