In a comment above you said that you're still using Snow Leopard on at least one machine and I can't recall if it works there (though I don't remember the last

In a comment above you said that you're still using Snow Leopard on at least one machine and I can't recall if it works there (though I don't remember the last time this feature changed and I've been using since 10.1)... Command-Shift-` will reverse the order Command-Tab will flip back and forth unless you exceed the delay while still holding command, at which point you can cycle through the stack. Holding shift reverses the direction. That being said, I agree that swapping back and forth in between windows of the same app is very useful and it's a pity that the two features aren't made to work in the same way. I'd like to do a quick Command-` to toggle two windows, or hold it down to see the list of windows for the current app.