Instead of using keyboard shortcuts, I like to use gestures for most of these kinds of actions in Xcode. To do this, simply slide 3 fingers up or down and wait a bit. It will switch between the header and implementation file. I find it works great on the Macbook Pro trackpad or my Magic Trackpad, which has enough space for these gestures. If you have a Magic Mouse, you may not have enough space for this and other gestures.
I think it will be easier for you to remember this gesture over keyboard shortcuts.
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