How to track (or see) expenses by category AND by credit card?

This is a perfect use case for tags. You would use one category of Clothes, but then tag each Clothes expenditure with either your name or her name. Then, you could run a report that included everything with in the Clothes category with either your name or her name as the tage. For more on tags, see here. For running reports, you could either use the Tags Report or the Transaction Tracker.

Tags are a critical part of my Tiller set-up. I use them for precisely the case you describe (to see how much my teenagers are spending on various things :smirk:) or, for example, to track all of the expenses related to a vacation across different categories so that I can easily see the total cost of the vacation.