can only add a single key combination to the "keyboard shortcut" applet

On Ubuntu Feisty -- and I presume all older variants -- each action in the Keyboard Shortcuts applet can have only single shortcut attached. (The program is "gnome-keybinding-properties".)

There should be at least a pair of primary/secondary shortcuts for each action. (See for example Amarok's global shortcuts.)

Example usage: I touch-type, and I usually have lots of shortcuts all over the keyboard that can be very easily typed when doing other work. However, many of them are hard to press one-handed, for example when I'm not working and I just want to skip a track. The reverse is also true: key combinations that are easy to press one-handed are unpleasant to use when touch-typing: these are touches close to the corners, and they need some finger-twisting to reach.