Page Up in tmux copy mode

Question

Using tmux 1.4 instead of using <prefix>PPage to switch to copy mode and navigate to previous pages, I would like to use PPage / NPage in a more natural way ":

• Switching the first page to copy mode -u
• On the next page up, move one page up, the page down will also move down.

Adding bind-key -n PPage copy-mode -u does the first, but not the second.

Any idea? (if this is useful, I use iterm2 on OSX)