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How separate individual expenses from family expenses in Gnucash?
In your words, you want to "easily determine whether an item was purchased as part of our individual accounts, or our combined family account." It's not clear exactly to me what kind of reporting you're trying to get. (I find a useful approach here to be to start with the output you're trying to get from a system, and then see how that maps to the input you want to give the system.) Here's some possibilities: