There are two ways actually!
1. Using the fabulous KeyboardShortcuts addin.
Obviously I use this one, I wrote it!
Simply assign the SWB of Ctrl+Tab and Ctrl+Shift+Tab to the Next/Prev Window menu items:
2. Using the built in Menu Shortcuts feature.
Hmm… actually I just assumed that you could but when I went to make a screenshot it appears that no, you cannot. Sorry!
No idea why they filter out the Next/Prev Window menu items. Is it just my machine because I have been using the KeyboardShortcuts addin?